feat: add support for relativising type exports

[danielroe]

Follow-up on #134.

This also includes a fix to respect the extension of the file for d.mts and d.cts files (#26).

[codecov]

Codecov Report

Merging #135 (346f884) into main (4892929) will increase coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head 346f884 differs from pull request most recent head c16d13e. Consider uploading reports for the commit c16d13e to get more accurate results

@@            Coverage Diff             @@
##             main     #135      +/-   ##
==========================================
+ Coverage   96.32%   96.33%   +0.01%     
==========================================
  Files           8        8              
  Lines         517      519       +2     
  Branches      100      101       +1     
==========================================
+ Hits          498      500       +2     
  Misses         19       19              

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

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