fix: add .js extension for relative import/exports in declarations

[danielroe]

Context: nuxt/nuxt#18426

Currently with Node16-style module resolution, the following will not be recognised by TS within a declaration file (imagine we have index.d.ts and other.d.ts):

export * from './other'

In order for this to be understood, it needs to have the JS file extension equivalent of the other dts file, which would be .js. (.mjs would be the equivalent for a .d.mts file).

Despite the fact that no .js file exists, TS understands the following to mean what we want:

export * from './other.js'

This PR adds support for a new flag (addRelativeDeclarationExtensions) which enables the addition of this extension within the outputted files. I've tested this within the Nuxt codebase both with and without Node16 style module resolution but it would benefit from broader testing as well, which is why I'm opening the PR for feedback at this point.

Remaining upstream todo would be to detect type import/exports within https://github.com/unjs/mlly.

[codecov]

Codecov Report

Merging #134 (3ae5952) into main (d512a61) will increase coverage by 0.22%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #134      +/-   ##
==========================================
+ Coverage   96.09%   96.32%   +0.22%     
==========================================
  Files           8        8              
  Lines         487      517      +30     
  Branches       98      100       +2     
==========================================
+ Hits          468      498      +30     
  Misses         19       19              

Impacted Files	Coverage Δ
src/make.ts	90.07% <100.00%> (+0.36%)	⬆️
src/utils/dts.ts	100.00% <100.00%> (ø)

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[danielroe]

@pi0 Might be worth merging #110 first; I'm happy to update this PR subsequently as it's not that complex.

[pi0]

Let's quickly move forward for this. Looking for mlly PR for type support. Also 110 probably needs extra care for possible breaking changes.