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

crates/theme_importer/Cargo.toml

@@ -7,7 +7,7 @@ license = "GPL-3.0-or-later"
 
 [dependencies]
 anyhow.workspace = true
-clap = { version = "4.4", features = ["derive"] }
+clap = { workspace = true, features = ["derive"] }
 gpui.workspace = true
 indexmap = { version = "1.6.2", features = ["serde"] }
 log.workspace = true