Rust: support non-ascii identifiers in syntax highliting

[Alphare]

rust-lang/rust#83799 has been stabilized in 1.53, which means that my first name is now a valid Rust identifier [1]. While this is great news, turns out that only rapha in raphaël is highlighted as a function name.

I am not sure if Javascript's regex engine has a way of easily defining the Unicode characters valid under this new feature, but I thought I'd at least open an issue.

[1] https://github.com/rust-lang/rfcs/blob/master/text/2457-non-ascii-idents.md#reference-level-explanation