package output as esm + cjs

[ivanhofer]

I need this package in an esm context so I took the time to convert it.
There weren't that many things to change, so the process was straigt forward.

• The build script now outputs cjs and esm code.
  In order to make the code run in an esm context without setting "type": "module" inside package.json all generated esm files need to end with the .mjs file extension. Therefor the fix-esm-output.js script is needed.
• All imports were altered using the "node16" module resolution.
• All require statements are rewritten using import statements.
  This did remove the "lazy-import" behavior as dynamic import statements must use async-await. But the only place where this affects regular runtime code is where esprima get's used. But since it is a regular dependency of recast nothing should fail.
• Tests were left unchanged and still run sucessfully for the cjs output.

I hope this could be merged into the main project so I don't have to maintain a copy of it.
Let me know if there are still some things missing.

[ivanhofer]

Some TypeScript related issues popped up when converting from require (which has no type support) to import. The issues are already present in the current master branch. I marked them with // @ts-expect-error.