Add gen keyword

[mohe2015]

As this is an edition 2024 keyword I don't know whether we want to do this. Because this will then also incorrectly highlight e.g. variable identifiers named gen in edition 2021

[Wilfred]

Thanks for the PR!

Based on rust-lang/rfcs#3513, I see it's reached final comment period with disposition to merge, so this seems reasonable to merge. I imagine that identifiers named gen are rare, and I see we're already highlighting edition specific keywords (e.g. dyn) and weak keywords (e.g. union), based on Keywords in the Rust Reference.