Add assetExtname replacement in entryFileNames

[BPScott]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers: #3107 #3116

Description

When using preserveModules by default rollup adds the original extension of non-js/ts files to the generated filename. E.g. foo.txt compiles to foo.txt.js (so that it doesn't clash with foo.js). This behaviour was added in #3116. This is great, however it is a little tricky to emulate when customising the output filename with entryFileNames.

If I wanted the same-as-default behaviour but outputting .mjs extensions I could pass a function to entryFilenames but it is pretty dang clunky:

{
  // ...
  preserveModules: true,
  entryFilenames: (chunkInfo) => {
    const NonAssetExtensions = ['.js', '.jsx', '.ts', '.tsx'];    
    const isAssetfile = !NonAssetExtensions.some((nonAssetExt) =>
      (chunkInfo.facadeModuleId || '').endsWith(nonAssetExt),
    );

    return `[name]${isAssetfile ? '[extname]' : ''}.mjs`;
  };

Instead I would like to propose adding an assetExtname replacement, that returns the extension prefixed with a . (same as extname) if the extension is not one of the "non asset extensions".

With this change, having rollup's default behaviour but outputting .mjs filenames can be done by setting entryFilenames to '[name][assetExtname].mjs'.

[github-actions]

Thank you for your contribution! ❤️

You can try out this pull request locally by installing Rollup via

npm install BPScott/rollup#asset-extname

or load it into the REPL:
https://rollupjs.org/repl/?pr=4077

[BPScott]

Question: I couldn't see any prior art for how to name multi-word replacements so I went with camel case for assetExtname. Is that acceptable?

[lukastaegert]

Looks good, makes sense to me. Bonus points for updating the documentation. Will need to wait for GitHub Actions to work properly again before merging, though.

[codecov]

Codecov Report

Merging #4077 (9d53b7c) into master (b460516) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4077   +/-   ##
=======================================
  Coverage   97.49%   97.49%           
=======================================
  Files         193      193           
  Lines        6824     6824           
  Branches     2006     2005    -1     
=======================================
  Hits         6653     6653           
  Misses         84       84           
  Partials       87       87           

Impacted Files	Coverage Δ
src/Chunk.ts	100.00% <100.00%> (ø)

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[BPScott]

Pushed a change to fix the linting error