Proposal: add rust-script support

[crabdancing]

rust-script, for those who aren't aware, is a popular mechanism by which Rust projects can be distributed as a single all-in-one shell script style file.

Its use cases are varied -- probably mostly sysops/devops, providing superior performance (after first run) to other solutions like Python & Bash, while inheriting the superior syntax and feature set of the full Rust language. It has the potential to bring the safety-first performant benefits of Rust to an entire new use case, making maintainable performant scripting a much easier problem to solve.

Unfortunately, the ecosystem is not very rust-script aware, which makes it a pain to use in practice. vscode-rust intellisense doesn't seem to work without an external Cargo manifest. Syntax highlighting doesn't work with the .ers extension. There are probably some other issues, but they aren't coming to mind at the moment.

[lnicola]

This extension is unmaintained. You want rust-lang/rust-analyzer#9609.

I suppose we could add .ers support, but running without a Cargo.toml isn't trivial.