Generate index.d.mts and index.d.cts when using mkdist entries.

[JesusTheHun]

Describe the feature

I'm compiling a lib for both CJS and ESM, with the following config :

import { defineBuildConfig } from 'unbuild';

export default defineBuildConfig({
  entries: [
    {
      builder: 'mkdist',
      input: './src/',
      outDir: './dist',
      ext: 'mjs',
      format: 'esm',
      declaration: true,
    },
    {
      builder: 'mkdist',
      input: './src/',
      outDir: './dist',
      ext: 'cjs',
      format: 'cjs',
      declaration: true,
    },
  ],
  clean: true,
  sourcemap: true,
});

As per the documentation, I expected declaration: true to generate 3 files : d.ts, d.mts and d.cts.
Yet, a single d.ts file is generated. I tried to move the declaration: true at the root, without success.

Additional information

• Would you be willing to help implement this feature?

[cloydlau]

This is very useful, as it would otherwise fail the @arethetypeswrong/cli detection.

It seems this PR can solve this, but don't know when it will land.