Fix string decoding for latin1, binary, ascii, base64 and base64url

[keichi]

Problem

sanitizeEncoding accepts ascii, latin1, binary, base64 and base64url, but generateString passes every non-hex encoding to new TextDecoder(encoding):

• binary, base64, base64url: TextDecoder doesn't support them, so parse() throws RangeError.
• latin1, ascii: WHATWG maps both to windows-1252, so byte 0x80 decodes to "€" instead of "\x80" (Node Buffer behavior).

Repro:

Parser.start().string("s", { length: 1, encoding: "base64" }).parse(Uint8Array.of(0x41));
// RangeError: The "base64" encoding is not supported
Parser.start().string("s", { length: 1, encoding: "latin1" }).parse(Uint8Array.of(0x80)).s;
// "€" (expected "\x80")

Fix

Handle these encodings the same way hex already is. Two small self-contained helpers are passed in through ctx.addImport. No Buffer is used, so it still works in browsers:

• latin1/binary: String.fromCharCode.apply over 8192-byte chunks (bytesToLatin1)
• ascii: clear the high bit of each byte in a loop, like Node, then UTF-8 TextDecoder, which is exact because every byte is now < 0x80 (bytesToAscii)
• base64: btoa(latin1); base64url: same with +/ mapped to -_ and padding removed (same as Node)

I skipped Uint8Array.prototype.toBase64 because Node 22 (the engine floor) doesn't ship it unflagged. btoa is available in every supported runtime.

generateString now builds the decode expression in one place for all four length modes, so the hex/TextDecoder ternary isn't repeated four times.

The README said "all encodings supported by TextDecoder", but the allow-list rejects things like utf-16le and shift_jis. I updated the README to list what's actually accepted. Widening the allow-list is a separate change.

Regression tests in test/primitive_parser.ts check every encoding against Buffer#toString() output, including a 20000-byte buffer that spans several chunks.

Decode time (Node 26, µs per call), per-byte Array.from vs this PR:

size	latin1 Array.from	latin1 chunked	ascii (.map + chunked)	ascii (loop + TextDecoder)
1 KB	20.2	2.8	7.1	0.8
64 KB	1293	155	447	39