Full ESM compliance

I think such primitive is a must for any filepath-related tasks, so I'm interested in making it portable to other than Node runtimes.
* Remove node-specific stuff.
* Make it ESM.
* Support import.meta.url as an initial.
* Test on deno, bun and maybe in browsers.

Right now it is pure CJS with typings, so it is still good and usable, but pure ESM would be better considering all above.