Repeated capture names aren't being flagged

[BekaValentine]

In a TSG pattern such as

(thing (_)@capture)@capture

there is a repeated capture name. This ought to be an error but currently doesn't seem to be, at least not at any level that manifests to cargo test.

[BekaValentine]

@robrix this might be a good feature to add to our test tooling that does query validation, etc.?