@nx/rollup: Allow outputting .mjs files for better dual format publishing

[TheHolyWaffle]

By default, the rollup config spits out cjs and esm type formats:

module.exports = withNx(
  {
    main: './src/index.ts',
    outputPath: './dist',
    tsConfig: './tsconfig.lib.json',
    compiler: 'swc',
    format: ['cjs', 'esm'],
  },
  {
    // Provide additional rollup configuration here. See: https://rollupjs.org/configuration-options
    // e.g.
    // output: { sourcemap: true },
  }
);

However, for esm dual publishing best practices. It's recommended for the esm files to have the .mjs file extension instead of .js. This is validated by running a tool like "are the types wrong" on the generated output.