Improve packaging strategy, native ES Modules, and browser-first builds

[justjake]

This pull request re-organizes the project into a collection of seperate NPM packages in a monorepo structure:

• packages/quickjs-emscripten - mostly backwards compatible all-in-one package.
• packages/quickjs-emscrpten-core - just the Typescript code, should be 100% browser compatible.
• packages/quickjs-browser-* - browser-compatible WebAssembly variants
• packages/quickjs-node-* - node-compatible WebAssembly variants

Many tool changes / improvements:

• Emscripten 3.1.50
• Yarn 4
• Mocha -> Vitest
• Vastly simplified the Makefile situation, although now there's a "meta build system" that produces variant-specific makefiles from a template. The new setup is more maintainable, and should allow us to add new variants like quickjs-ng much more easily than the old system.

Fixes #121, #117, #96, #33