Support for standalone .rs files without Cargo.toml

[sohang3112]

I want to try using Rust for short scripts (similar to scripting in Bash or Python). Creating a full-fledged Cargo project can be overkill for such tasks.

I can compile & run these standalone scripts using rustc script.rs. It would be nice if Rust Analzyer also supported such scripts (for example, showing the Run button above fn main() in VS Code).

[poliorcetics]

Especially with cargo script in nightly now, this will quickly become very very useful

[Veykril]

Duplicate of #14318

Also just to note, for cargo script support we'd also need to first see support for cargo metadata parsing cargo scripts