Rspack support

[ngranander]

Adds Rspack as an alternative bundler for webpack_bundle and webpack_devserver, addressing #159.

A new bundler parameter ("webpack" or "rspack", default "webpack") controls which bundler is used. When set to "rspack", the rules use @rspack/core and @rspack/cli instead of webpack and webpack-cli, via a new rspack_binary macro that mirrors the existing webpack_binary.

Config merging:
Rspack CLI doesn't support --merge, so when a custom webpack_config is provided it replaces the generated base config entirely. Without a custom config the generated base config is used as-is. This is documented in the bundler param docstrings and the README.

Performance:
Rspack's Rust-based architecture provides significant speedups over webpack. In published benchmarks rspack is 4-6x faster for production builds and up to 15x+ faster for dev startup on larger projects. In a real-world production web application (~10k modules with SWC loaders, CSS extraction, and code splitting), we measured approximately 2.3x (dev) and 3.2x (prod) end-to-end Bazel build speedup.

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Changes are visible to end-users: yes

• Searched for relevant documentation and updated as needed: yes
• Breaking change (forces users to change their own code or config): no
• Suggested release notes appear below: yes

New: webpack_bundle and webpack_devserver now support Rspack as a drop-in alternative bundler. Set bundler = "rspack" and ensure @rspack/core and @rspack/cli are linked in your node_modules. See the README for details.

Test plan

• Covered by existing test cases
• New test cases added
  • webpack/tests/simple: rspack with custom config and with generated config
  • webpack/tests/devtool-override: rspack with devtool override
  • webpack/tests/worker: rspack with workers enabled
  • webpack/tests/devserver: rspack devserver target
  • e2e/smoke: rspack end-to-end smoke test with production-mode output comparison

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Test

1 test target passed

Targets

//docs:update_test [k8-fastbuild] 102ms

Total test execution time was 102ms. 19 tests (95.0%) were fully cached saving 2s.

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e2e/loaders

All tests were cache hits

1 test (100.0%) was fully cached saving 44ms.

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e2e/loaders_jslib

All tests were cache hits

1 test (100.0%) was fully cached saving 38ms.

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Test

e2e/smoke

All tests were cache hits

3 tests (100.0%) were fully cached saving 173ms.

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Test

e2e/smoke_rjs3

All tests were cache hits

1 test (100.0%) was fully cached saving 45ms.

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Test

e2e/worker

All tests were cache hits

2 tests (100.0%) were fully cached saving 124ms.

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