feat: added optional support for colorized output

[fredbi]

This PR adds support for colorized output, with minimal extra dependencies.

• Actual / Expected in equal colorized
• Diff output is colorized

It is enabled with the specific extra blank import:

_ "github.com/go-openapi/testify/enable/colors/v2"

When enabled, colorized output is:

• enabled by go test flag -testify.colorize
• or by environment variable TESTIFY_COLORIZE=true

By default, colors are chose to be rendered on a dark terminal.

You may use darker colors on a bright terminal with:

• go test flag -testify.colorize -testify.theme=light
• or by environment variable TESTIFY_THEME=light

This work is inspired and adapts the following PRs:

• github.com/Colorizing test output stretchr/testify#1467
• github.com/assert: add option to colorize diffs produced by assert.Equal stretchr/testify#1480
• github.com/#946 Colorize 'Expected' and 'Actual' in Assert Package + Minor Fixes: #1112, #1086 & #1231 stretchr/testify#1232
• github.com/Colorize 'expected' and 'actual' stretchr/testify#994

Change type

Please select: 🆕 New feature or enhancement|🔧 Bug fix'|📃 Documentation update

Short description

Fixes

Full description

Checklist

• I have signed all my commits with my name and email (see DCO. This does not require a PGP-signed commit
• I have rebased and squashed my work, so only one commit remains
• I have added tests to cover my changes.
• I have properly enriched go doc comments in code.
• I have properly documented any breaking change.

[Copilot]

Pull request overview

This PR adds optional colorized output support to testify assertions with minimal dependencies. The feature is opt-in via a blank import and can be controlled through CLI flags or environment variables.

Changes:

• Refactored difflib package to support customizable output printers through an Options system
• Added colorization support for assertion failures (expected/actual values and diff output)
• Implemented theme support (dark/light) for different terminal backgrounds

Reviewed changes

Copilot reviewed 29 out of 30 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
internal/difflib/options.go	New Options system for customizable diff output printers
internal/difflib/options_test.go	Tests for colorized diff output
internal/difflib/matcher.go	Extracted SequenceMatcher types and logic from difflib.go
internal/difflib/difflib.go	Refactored to use the new Options system
internal/difflib/difflib_test.go	Improved test organization
internal/difflib/difflib_benchmarks_test.go	Separated benchmarks into dedicated file
internal/assertions/diff.go	Moved diff logic with integrated color support
internal/assertions/equal.go	Updated to colorize expected/actual values
internal/assertions/helpers.go	Removed code moved to diff.go
internal/assertions/enable/colors/*.go	Internal colorization API and implementations
enable/colors/*.go	Public colorization module with CLI/env integration
assert/enable/colors/enable_colors.go	Public indirection API for colorization
docs/doc-site/examples/EXAMPLES.md	Added colorization documentation and examples
README.md	Updated to mention colorized output feature
go.work	Added enable/colors module to workspace

---

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[Copilot]

The comment suggests checking if t is not nil, but this check is actually unnecessary in Go. reflect.TypeOf() will never return nil - if v is nil, reflect.TypeOf(v) returns a non-nil reflect.Type that represents the nil type. The Kind() method can safely be called on this value. This comment should be removed as it suggests an enhancement that isn't needed and might confuse readers.

Suggested change

	k := t.Kind() // Proposal for enhancement: check if t is not nil
	k := t.Kind()

[fredbi]

reflect doc: TypeOf returns the reflection Type that represents the dynamic type of i. If i is a nil interface value, TypeOf returns nil.

[codecov]

Codecov Report

❌ Patch coverage is 60.09852% with 162 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.63%. Comparing base (374235c) to head (bdf9d89).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
internal/assertions/enable/colors/themes.go	0.00%	36 Missing ⚠️
internal/difflib/matcher.go	80.83%	21 Missing and 11 partials ⚠️
internal/assertions/enable/colors/options.go	0.00%	25 Missing ⚠️
internal/assertions/enable/colors/enable_colors.go	0.00%	23 Missing ⚠️
assert/enable/colors/enable_colors.go	0.00%	12 Missing ⚠️
internal/assertions/enable/colors/colors.go	33.33%	11 Missing and 1 partial ⚠️
internal/difflib/difflib.go	80.48%	0 Missing and 8 partials ⚠️
internal/difflib/options.go	81.81%	4 Missing and 2 partials ⚠️
internal/assertions/diff.go	87.17%	4 Missing and 1 partial ⚠️
internal/assertions/equal.go	75.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #33      +/-   ##
==========================================
- Coverage   88.76%   87.63%   -1.13%     
==========================================
  Files          63       71       +8     
  Lines        8436     8598     +162     
==========================================
+ Hits         7488     7535      +47     
- Misses        782      892     +110     
- Partials      166      171       +5     

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