fix: rewrite .ts to .js when rewriteRelativeImportExtensions enabled (svelte-package...

fix: rewrite .ts to .js when rewriteRelativeImportExtensions enabled (svelte-package) (#14936)

* add a failing test

* js all the way

* implem

* add changeset

* reuse import regex, align logic with rewriteRelativeImportExtensions setting

* perf: cache tsconfig lookup

* add rewriteRelativeImportExtensions

* consolidate tests

* tweak

* Update .changeset/fresh-pants-camp.md

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Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>
Co-authored-by: Simon Holthausen <simon.holthausen@vercel.com>