Add cargo xtask clippy (#8722)

This PR sets up a `cargo xtask clippy` command for running `cargo
clippy` with our defined set of options.

The intent is to make this easier to manage as we start enabling more
Clippy rules.

Release Notes:

- N/A

script/clippy

@@ -2,8 +2,4 @@
 
 set -euxo pipefail
 
-# clippy.toml is not currently supporting specifying allowed lints
-# so specify those here, and disable the rest until Zed's workspace
-# will have more fixes & suppression for the standard lint set
-cargo clippy --release --workspace --all-features --all-targets -- --allow clippy::all --deny clippy::dbg_macro --deny clippy::todo
-cargo clippy --package gpui -- --deny warnings
+cargo xtask clippy