docs: add Windows build instructions, WSL recommendation, Node version requirements, and troubleshooting

[VARUN15VS]

Summary

This PR improves the README by adding clear, detailed documentation for Windows users who experience build failures when running npm install or python build.py.

Building scratch-blocks currently fails on native Windows due to long command-line limits, deprecated Closure Compiler tooling, Python 2 syntax, and modern Node.js incompatibilities. These issues are well-documented but not yet mentioned in the README.

What This PR Adds

• A new section: Building Scratch Blocks on Windows (Known Issues & Workarounds)
• Recommended workflow using WSL 2
• Node.js 8–10 requirement
• Python 2 requirement (build.py uses Python 2 syntax)
• Step-by-step instructions to build inside WSL 2
• Troubleshooting section for common Windows errors
• Reference to Issue Windows: Some build steps fail with "The command line is too long" #2080 (“The command line is too long”)

Why This Is Needed

Many contributors on Windows encounter errors like:

The command line is too long
UNKNOWN ERROR
Module not found: Error: Can't resolve '../blockly_compressed_horizontal'

These issues are still present and reproducible.
Adding this documentation helps developers avoid failed builds and guides them toward the officially recommended WSL workflow.

Related Issues

• Documents the problem reported in
  Windows: Some build steps fail with "The command line is too long" #2080

Notes

This PR does not modify any existing information — it only appends new documentation sections that help Windows users build the project successfully.

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[VARUN15VS]

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