TextDecoder is wrong and very slow

## Correctness Encodings that return invalid results: * Single-byte: * `ibm866` (fails at even ascii input) * `koi8-u` * `windows-874` * `windows-1252` * `windows-1253` * `windows-1255` * Multi-byte (all except `gb18030`): * `gbk` (should be identical to `gb18030` but it is instead broken) * `big5` * `euc-jp` * `iso-2022-jp` * `shift_jis` (fails at even ascii input) * `euc-kr` Unimplemented encodings that throw: * `iso-8859-16` * `x-user-defined` If built without `icu`, `utf-16le` encoding also returns invalid results: ```js > new TextDecoder('utf-16le').decode(Uint16Array.of(0xd800)) '�' // correct '\ud800' // no ICU ``` ## Performance * `utf-8` (aka default) `TextDecoder` is much slower on ascii input than it can and should be `1.3x` on 4096 bytes, `~3x` on 1 MiB input * The above applies to `buffer.toString()` too It's much slower on ASCII input than a checked js impl (same `1.3x`-`3x`) * `windows-1252` aka `new TextDecoder('ascii')` aka `new TextDecoder('latin1')` is ~`2x-4x` slower than an optimized impl on ascii input * `windows-1252` aka `new TextDecoder('latin1')` is ~`6x-12x` slower than an optimized impl on latin1 input * `windows-1252` is ~`7x-12x` slower than an optimized js impl * Other single-byte encodings that are significantly slower than js impl even on non-ascii input: `iso-8859-3`, `iso-8859-6`, `iso-8859-7`, `iso-8859-8`, `iso-8859-8-i`, `windows-1253`, `windows-1255`, `windows-1257` * None of the single-byte encodings are _faster_ than the js impl even on non-ascii input * All of the single-byte encodings except `windows-1252` are `>=10x` slower than the js impl on ascii input (`windows-1252` is _only_ ~`2-4x` slower) ## References Nothing of the above requires any changes on the native side, I compared to a somewhat optimized JS implementation See https://docs.google.com/spreadsheets/d/1pdEefRG6r9fZy61WHGz0TKSt8cO4ISWqlpBN5KntIvQ/edit See tests in https://github.com/ExodusOS…