fix: relative declaration extension should match the extension of the code output

[cjpearson]

Makes sure the extension used in the import inside the declaration file matches that of the output. When the output code is uses .mjs the import in the declaration should look like ./foo.mjs when addRelativeDeclarationExtensions is enabled.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.69%. Comparing base (9000888) to head (1598b8d).
Report is 77 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #272      +/-   ##
==========================================
- Coverage   82.86%   80.69%   -2.18%     
==========================================
  Files          12       13       +1     
  Lines         852      896      +44     
  Branches      133      190      +57     
==========================================
+ Hits          706      723      +17     
- Misses        144      171      +27     
  Partials        2        2              

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