Support Rust builds of Fish shell

[Matthew-Benson]

Starting with Fish 3.8.0, the official release of Fish shell is built with Rust primarily. With 3.8.0 not having released at the time of writing this, the build/install process for this has not been finalized and work is currently ongoing in fish-shell/fish-shell#10367.

We will need to use the rust toolchain from rules_rust to build this. It should be relatively simple, as I believe Fish is planning to just build the binary with cargo build, but they intend to maintain cmake to some extend for installation. This may change though since this is all up-in-the-air.

If we need, we may be able to generate a native build dynamically using Gazelle with gazelle_rust to make build targets based on project imports, etc.

[faho]

the build/install process for this has not been finalized and work is currently ongoing in fish-shell/fish-shell#10367.

Note: That PR is for a separate option to build/install fish.

We have no plans or inclination to drop the regular build or promote this one to be the main thing. It has some awkward downsides.

You want to follow fish-shell/fish-shell#10181 instead.