Support `rewriteRelativeImportExtensions` in `@sveltejs/package`

[colinlienard]

Describe the problem

With the recent release of typescript 5.7 and the arrival of the rewriteRelativeImportExtensions setting, it would be nice if svelte-package could support it.

Describe the proposed solution

Rewrite .ts extensions to .js if rewriteRelativeImportExtensions is present in the tsconfig

Alternatives considered

Currrently using a script to do so

Importance

nice to have

Additional Information

Explanation of the setting: https://devblogs.microsoft.com/typescript/announcing-typescript-5-7-rc/#path-rewriting-for-relative-paths

[ryanatkn]

The option appears to work for relative paths, but not for aliased paths. I tried a fix in #14673