[Feature]: Provode .mjs artifact

[chenjiahan]

What problem does this feature solve?

Provode .mjs artifact for ESM imports.

What does the proposed API look like?

Both CommonJS and ESModule should be supported.

[chenjiahan]

@10Derozan cc

[fi3ework]

Quotes from 1.0 roadmap

Before the release of v1.0, we need to finish the ESM support for all packages

Given Rsbuild is a quite new project, we have more choices on this topic based on Vite's valuable experience.

1. Only ship ESM dist to NPM. This might be too bold as we have given up users who can't migrate to ESM project.
2. Only ship async API from rebuild and @rsbuild/shared in CJS version. API is for upper-level framework, so this won't break anything as the ecosystem will grow up after 1.0, also this could push the upper-level to embrace ESM. When we migrate to ESM only in the future, we won't blocked from dropping sync API support.

Also, there's a risk of loading duplicated modules from ESM and CJS from the same source module distributed in CJS and ESM. A thin ESM wrapper to support ESM would be a more secure method.

[chenjiahan]

It would be very nice if we could use the ESM wrapper to keep compatible with CJS users.

Add some information for this:

• Rspack does not yet have full ESM loader support ([Feature]: Support native ESM loader rspack#5735)
• Our packages have exported some constants (such as version), which may not be converted to async API.
• All Modern.js packages are still use the CJS API..