Deals with https://github.com/zed-industries/community/issues/1282

If rust-analyzer responds with non-empty message, opens a new singleton
multibuffer with the expansion result:

![image](https://github.com/zed-industries/zed/assets/2690773/5f790ffd-1ad8-4f02-a60d-49ee0756362d)

In gpui2, only editors with excerpts containing Rust languages get the
action, in gpui1 every editor (even the *.ts one) has this action
enabled but it does nothing.

Release Notes:

- Added an `editor::ExpandMacroRecursively` command to expand Rust
macros with rust-analyzer.