fix: declaration extensions should correspond to their js extension

[cjpearson]

This is a partial fix for #138. (Wasn't sure if I should split it out into a new bug) It addresses the issue where changing the extension of the output does not affect extension of the declaration so there is a mismatch. It does not implement automatically choosing the extension based on the source file.

The other important thing is that .ts, .cts, .mts on the declaration files imply the existence of the file with the same basename: that is, index.d.ts says ‘the declaration file is for index.js’, a.d.cts is ‘for a.cjs’, and b.d.mts is ‘for b.mjs’.

https://www.typescriptlang.org/docs/handbook/modules/reference.html#file-extension-substitution

The change itself is relatively straightforward, but it does have a large impact, especially because mjs is the default extension for esm output. Perhaps a major bump would be necessary or hiding the behavior behind a flag?

[cjpearson]

In this case there is an existing foo.d.ts in the source, but the generated foo.d.mts will be used instead. Would it be good to copy foo.d.ts and rename it? The developer could rename it to foo.d.mts in the src dir, but that would not work if they are generating both cjs and esm.

[cjpearson]

In this case we have a test2.cts in the src. I think there are two ways to handle it:

1. When a .cts or .mts file is found in the source, generate .cjs/.d.cts or .mjs/.d.mts pairs regardless of the ext option. This is the issue described by Declarations not genered to .mts and .cts files #138. Another variation would be to only override ext if it were not explicitly set.
2. Always use the ext option and treat .mts, .cts, and .ts files in src/ identically.

It's just important that the declaration and js file match, so you don't end up with foo.cjs and foo.d.mts

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 83.84%. Comparing base (9000888) to head (a68becc).
Report is 101 commits behind head on main.

Files with missing lines	Patch %	Lines
src/loaders/js.ts	75.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #268      +/-   ##
==========================================
+ Coverage   82.86%   83.84%   +0.97%     
==========================================
  Files          12       12              
  Lines         852      953     +101     
  Branches      133      197      +64     
==========================================
+ Hits          706      799      +93     
- Misses        144      153       +9     
+ Partials        2        1       -1     

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[productdevbook]

@danielroe light ping