LibTextCodec/Latin1: Iterate over input string with u8 instead of char

Using char causes bytes equal to or over 0x80 to be treated as a
negative value and produce incorrect results when implicitly casting to
u32.

For example, `atob` in LibWeb uses this decoder to convert non-ASCII
values to UTF-8, but non-ASCII values are >= 0x80 and thus produces
incorrect results in such cases:
```js
Uint8Array.from(atob("u660"), c => c.charCodeAt(0));
```
This used to produce [253, 253, 253] instead of [187, 174, 180].

Required by Cloudflare's IUAM challenges.

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