Fix String with escapes prefix matching on Erlang.

[aznashwan]

Fixes #3126.

Fixes String prefix matching on Erlang when the prefix contains escape characters or unicode codepoints by ensuring the correct prefix byte length is matched against.

[lpil]

Hello! Thank you for this

Could you reuse the code here please? I would like to avoid having two different bits of code for this 🙏

gleam/compiler-core/src/javascript/pattern.rs

Line 856 in 2c59b9c

fn utf16_no_escape_len(str: &EcoString) -> usize {

It'll need to be make slightly more flexible to specify whether it's utf8 (Erlang) or utf16 (JavaScript)

[aznashwan]

Could you reuse the code here please?

Done, hadn't noticed it before since I only investigated the Erlang part of the compiler tree but it was basically exactly what I needed.

I've adapted the function to directly return the updated string, properly treat the escape chars Gleam supports, and moved it to a top-level crate::strings module for lack of a better place to put it, but please feel free to suggest a new one.

I've also split the specific prefixes tested against to make the transpiled code a little easier to follow.

[lpil]

Lovely! Thank you.