Initial Rust highlighting support
* Basic support for the language
* Line comments (`//`)
* Block comments (`/* */`), but no recursion
* Chars
* Strings (`"..."`)
* Raw strings (`r"..."`, `r#"..."#`, up to 5 levels)
* Added some Rust syntax test files
Co-authored-by: Romain Failliot <romain.failliot@foolstep.com>
Alexander Lopatin