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 88.88889% with 1 line in your changes missing coverage. Please review.

Project coverage is 80.29%. Comparing base (9000888) to head (bd52c24).
Report is 77 commits behind head on main.

Files with missing lines	Patch %	Lines
src/loaders/js.ts	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #268      +/-   ##
==========================================
- Coverage   82.86%   80.29%   -2.58%     
==========================================
  Files          12       12              
  Lines         852      888      +36     
  Branches      133      190      +57     
==========================================
+ Hits          706      713       +7     
- Misses        144      173      +29     
  Partials        2        2              

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[Teages]

The declaration file of vue is special, perhaps it should be .vue.d.ts?

I did some test and it looks like typescript cannot read the types from *.vue.d.mts when importing *.vue

[cjpearson]

Good point, but it looks like it should actually be .d.vue.ts https://www.typescriptlang.org/tsconfig/#allowArbitraryExtensions

[Teages]

Declaration for vue has a longer history than this flag, and the most of vue community is currently using .vue.d.ts.

I think it would be fine that just skip vue declaration. So that we can automatic switch to .vue.d.ts if the community decides to switch because we are also using vue-tsc to generate vue declaration

[cjpearson]

Yes, I completely agree. I'll make sure those files stay untouched as part of this change and follow up with a separate issue.

[cjpearson]

#270