Support rewriteRelativeImportExtensions (TS 5.7+)

[michaelfaith]

TS 5.7 introduced the new rewriteRelativeImportExtensions option, which allows devs to write imports with .ts extensions, and it'll be transformed to .js extensions at compile time. Despite having the option set, tsup doesn't do the rewrites. Ideally it would rewrite the extension according to the module type emitted. e.g. esm with mjs extensions would rewrite the relative import extension to mjs.

Ref: https://devblogs.microsoft.com/typescript/announcing-typescript-5-7/#path-rewriting-for-relative-paths