Attempt to use the builtin and switch to Uint8Array

[domenic]

I got stuck on nodejs/node#40091 making it hard to write tests for the error case, so I'm abandoning this for now, but uploading it as reference.

From some brief testing it doesn't look like we got that particular error case right in the past, either, although in a different way.

This would solve #7. Although we'd probably want to explain the unusual architecture of the package in the readme a bit more.

This also adds support for the fatal mode from the spec; previously I believe we were implicitly using replacement. Probably this is useful for work like #11.

[ThisIsMissEm]

Does this block #11 or just improve the situation overall?

[domenic]

Anything that causes the tests to pass would help with #11. It looks like this both helps and hurts; it supports more encodings (i.e., more subdirectories which that patch hasn't even started on supporting) but it causes some failures for basic stuff like Big5.