Support `rewriteRelativeImportExtensions` (TS 5.7+)

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

No source file or test is named. Start by reading the TypeScript 5.7 path-rewriting reference and tracing tsup's handling of TypeScript options and emitted module extensions. Done means relative imports using .ts extensions are rewritten to the appropriate emitted extension, including mjs for the stated ESM case.

Written by the indexing model from the issue text.