Switch dprint to new gofumpt Wasm plugin

[jakebailey]

This should eliminate the performance woes of formatting with exec + go tool gofumpt (especially on Windows).

Benchmark 1: dprint fmt --incremental=false (branch = main)
  Time (mean ± σ):     42.703 s ±  0.412 s    [User: 0.733 s, System: 0.569 s]
  Range (min … max):   41.742 s … 43.399 s    10 runs
 
Benchmark 2: dprint fmt --incremental=false (branch = jabaile/gofumpt-plugin)
  Time (mean ± σ):     633.1 ms ±  14.8 ms    [User: 4887.2 ms, System: 1083.9 ms]
  Range (min … max):   617.3 ms … 663.2 ms    10 runs
 
Summary
  dprint fmt --incremental=false (branch = jabaile/gofumpt-plugin) ran
   67.45 ± 1.70 times faster than dprint fmt --incremental=false (branch = main)

Not in this PR is removing the tool, since we still format generated files. That will be a more invasive change.

[Copilot]

Pull request overview

This PR modernizes the Go code formatting setup by replacing the exec-based dprint plugin (which runs go tool mvdan.cc/gofumpt as an external command) with a dedicated WebAssembly-based gofumpt plugin. This change should significantly improve formatting performance, especially on Windows where process spawning is more expensive.

Key Changes:

• Replaced exec plugin configuration with native gofumpt Wasm plugin
• Configured langVersion to "go1.25" to match the project's Go version
• Set modulePath to the project's module identifier