Rollup merge of #139843 - thaliaarchi:editor-file-associations, r=Mark-Simulacrum

Setup editor file associations for non-rs extensions

.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`.

I have tested this with VS Code and Zed. I have not implemented it for Emacs/Eglot or Helix.

Author: ChrisDenton