rust.kak: Support Rust's raw-string syntax.
rust.kak: Support Rust's raw-string syntax.

According to the Rust language reference[1], a raw string starts with an 'r',
zero or more '#' characters, and a '"', and doesn't close until a '"' is
immediately followed by the matching number of '#' characters.

[1]: https://doc.rust-lang.org/reference/tokens.html#raw-string-literals

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