Rewrite base64 + base16 codecs using native APIs

TIL, the web platform has built in conversion between base64/hex strings and Uint8array

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint8Array/fromBase64
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint8Array/toBase64
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint8Array/fromHex
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint8Array/toHex

All are in Baseline 2025, ie available in the latest version of all browsers

Should make our base64 + base16 codecs much simpler + probably more performant

- We may want to wait until Node 26 releases, since it is only in 25 and that will be the next LTS
- Will need to use `EsNext.TypedArrays` lib in typescript 6.0 https://github.com/microsoft/TypeScript/pull/61696/files