This reduces the amount of time it takes to do the x check for
rust-analyzer analysis from 12m16s to 3m34s when the bootstrap compiler
is already downloaded.
.gitattributes lists *.fixed, *.pp, and *.mir as file extensions which
should be treated as Rust source code. Do the same for VS Code and Zed.
This only does syntax highlighting, which is appropriate, as MIR isn't
really Rust code.
At the same time, consistently order `rust-analyzer.linkedProjects`
between editors. For some reason, Eglot didn't include
library/Cargo.toml.
bjorn3