Switch CommonJS transform to cjs-module-lexer

[developit]

Now that cjs-module-lexer is battle-tested (it powers CommonJS in Node as of 14.17), we should switch to it. This gives us a clean way to support CJS-->ESM transformations like we're already doing, but without the horrible regex-based implementation I cobbled together.

• inline my forked copy of cjs-module-lexer's JS version with require() tracking
• create a new lib/transform-cjs.js module that wraps cjs-module-lexer (similar to lib/transform-imports.js)
• replace fast-cjs-plugin.js implementation with a call to the new library
• add/improve CommonJS tests
  (improved in Fix require()() in production bundle #156)

[developit]

Looks like cjs-module-lexer won't land my require() tracking fix:
nodejs/cjs-module-lexer#10

I published the dist files (both JS and WASM) to a branch which we can vendor.