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

[jycouet]

Fixes sveltejs/cli#783

transform .ts extensions to .js in import/export statements when using allowImportingTsExtensions

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[dummdidumm]

Thank you! Two suggestions:

• In utils.js#resolve_aliases we already have battle-tested regexes for replacing paths. We should extract that to reuse it here
• After doing that I think we can replace the e2e test with a unit test (check that the method replaces .ts->.js) as that one will run a lot faster (the e2e tests are really slow)

Update: With #14673 merged we also gotta take this into account. Will update this PR shortly

[dummdidumm]

thank you!