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
Before the change to `script/clippy`, bash ignored first `clippy`
invocation failure and CI moved on with Linux errors and warnings
emitted.
Release Notes:
- N/A
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Co-authored-by: Mikayla Maki <mikayla@zed.dev>