Support wasm32-unknown-emscripten

[logan-gatlin]

New emscripten build pipeline:

• Added --emscripten CLI flag to BuildOptions.
• New emscripten-specific post-build flow:
  1. Run emcc --post-link on the wasm-bindgen output to produce the emscripten JS runtime and post-linked wasm binary.
  2. Parse library_bindgen.js to extract exported functions and classes.
  3. Generate a thin ESM wrapper (emscripten_wrapper.js) that lazily initializes the emscripten module and exports Workers-compatible handlers + proxy classes for Durable Objects.
  4. Bundle with esbuild (wasm kept external).
  5. Clean up intermediate artifacts.
• Added check_for_emcc() with auto-detection of EMSDK environment variable to prepend emscripten tools to PATH.

New JS template (worker-build/src/js/emscripten_wrapper.js)

Thin ESM module that imports the emscripten factory + wasm module, provides a custom instantiateWasm hook for Workers wasm loading, and exports handlers/classes via template placeholders.

[codspeed-hq]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 2 untouched benchmarks

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_{Comparing emscripten (fc58343) with main (2d022ab)}