[code-infra] Support JSX and TS in bundle scripts

[Janpot]

Allow JSX and TS in custom bundle scripts. Also add a method for resolving package.json files in case no /package.json export is available and the user is on node 22.14+.

Confirmed in base ui that this allows for entries such as

{
  id: 'custom',
  code: `
    import { Field } from '@base-ui-components/react/field';
    export default <>
      <Field.Root />
      <Field.Label />
    </>
  `,
  externals: ['react', 'react-dom'],
}