Rust syntax highlighting compatibility with rust-script-style scripting.

[crabdancing]

When using rust-script, it's possible to write a shell-style script in Rust code, using a hashbang at the top. This is nifty, but when working in Helix, the syntax highlighting is incorrect for this use case:

With a small adjustment, it should work well enough that an entirely new workflow for working in Rust as a high-performance and type safe alternative to conventional scripting, would become more feasible.

Note that if I comment out the hashbang, the rest of the code now has correct highlighting:

This use case is becoming popular, to the point that there has been an experimental cargo RFC out for an essentially identical use case upstream (although it's still feature gated):

https://github.com/rust-lang/rfcs/blob/master/text/3424-cargo-script.md

Having syntax highlighting that can correctly ignore the initial hashbang (regardless of target), would fix this for rust-script, while being backward compatible with the early cargo-script project, and future-proof for upstream's cargo script.