CLI interface to rust-analyzer

I think at some point we had something like rust-analyzer complete /path/to/file.rs:line:column. I've think we've lost that at some point as not too useful, but, looking at https://users.rust-lang.org/t/how-many-symbols-can-rust-analyzer-autocomplete-in-practice/81400, perhaps we need to bring it back?

I think the way we want to do that is to allow specifying ra-commands inline in file. Ie, the CLI is

$ rust-analyzer complete ./src/foo.rs

and foo.rs contains something like

    let x = /*ra:complete*/

🤔 i guess there's also an argument that we should make the syntax exactly like the syntax of our tests? Ie, today tests say just $0, and its up to the context to figure out what command should we execute at $0, but I can imagine a universal check where you write just $0:complete$, $0:gotodef, etc.

Start by examining the proposed rust-analyzer complete ./src/foo.rs entry point and the existing test syntax that uses $0. Compare the inline /*ra:complete*/ proposal with command-specific markers such as $0:complete$ and determine the command semantics and test coverage needed for a usable CLI interface.