ci: Ignore `syntect` in the WASM example machete check (#2693)

## Summary

`cargo machete` on `main` fails with `syntect` reported as unused in
`gpui-base-examples-wasm`.

The dependency is used, but only from
`crates/base/examples/showcase/syntect_highlighter.rs`,
which the WASM example pulls in with `#[path =
"../../showcase/mod.rs"]`.
cargo-machete only scans the crate's own directory, so it cannot follow
that
path — this is the same reason `gpui-base` is already in the ignore
list.

## Test Plan

- `cargo machete`

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>