Support C string literals highlight

[rhysd]

Recently Rust has stabilized C string literals syntax (the tracking issue). It will be available on the stable toolchain soon.

constHELLO:&core::ffi::CStr = c"Hello, world!";constHELLO:&core::ffi::CStr = cr#"Hello, world!"#;

This PR adds support for the syntax.

Before:

After: