[Feature]: align with webpack: JavascriptModulesPlugin.getChunkFilenameTemplate

[Jack-Works]

What problem does this feature solve?

Missing but used API in https://github.com/awesome-webextension/webpack-target-webextension/

What does the proposed API of configuration look like?

https://github.com/webpack/webpack/blob/eda2e284ac7020fb8e8aa5a6c1fd1547a0ad1b08/lib/javascript/JavascriptModulesPlugin.js#L564

[kakiuwang-ui]

I opened #15280 implementing this.

It adds JavascriptModulesPlugin.getChunkFilenameTemplate(chunk, outputOptions) with the same resolution order as rspack's own get_js_chunk_filename_template in crates/rspack_core/src/options/output.rs:

1. chunk.filenameTemplate, when the chunk carries its own template
2. output.filename, when the chunk can be initial
3. output.chunkFilename otherwise

Everything it needs is already exposed to JS (chunk.filenameTemplate and chunk.canBeInitial() on the napi Chunk, plus compilation.outputOptions), so it is a pure TypeScript addition — no binding or Rust change.

One deliberate difference from webpack: webpack has a fourth branch returning output.hotUpdateChunkFilename for HotUpdateChunk instances. Rspack keeps hot update chunks inside the Rust HMR pipeline and never surfaces them to JavaScript, so no chunk reachable from a plugin can be a hot update chunk, and there is no JS-observable equivalent of ChunkKind::HotUpdate to branch on. I documented that rather than guessing — if the team would rather expose the chunk kind and keep the shape identical to webpack, I'm happy to change it.

The test covers all three branches from a single entry and additionally asserts that compilation.getPath(template, { chunk }) is one of chunk.files for every chunk, so it verifies the returned template is the one rspack actually rendered with instead of just restating the implementation.

Since this issue is still pending triage: if you'd prefer a different shape, or don't want this API surface at all, let me know and I'll adjust or close the PR.

@Jack-Works this should cover the webpack-target-webextension use case you linked.