File path completion for `include!` and friends

Hi, I know rust-analyzer has some support for include! (like go-to-definition) and I was wondering it it might also be possible to add completions for the actual file paths when the cursor is inside an include_* macro invocation.

This request is inspired by VSCode's markdown completion, which starts completing file paths if you start typing a path, e.g. [](./):

I found their initial implementation as well in case that's helpful: https://github.com/microsoft/vscode/commit/a4e529c759fae0c15bb4bb8817ac3e7428793164

I am not sure if this is something that makes sense for rust-analyzer to do, but I would guess it might be feasible if there is already special handling for the include_* macros? If this seems out of scope for r-a to implement, that's totally fine too, but I thought it seemed like a small convenience that might be handy in some cases (e.g. include_str!ing testdata)

Thank you! R-a is consistently my favorite language server across multiple languages, and I really appreciate all the work people have put into it!

Start by locating rust-analyzer's existing go-to-definition or special handling for include_* macros and the completion entry point used inside macro invocations. Review the linked VSCode implementation for path-completion behavior. Done means file paths are offered while typing inside include! and related macros, with coverage for the intended completion cases.