More fixes to the semantic index's chunking (#11376)

This fixes a tricky intermittent issue I was seeing, where failed to
chunk certain files correctly because of the way we reuse Tree-sitter
`Parser` instances across parses.

I've also accounted for leading comments in chunk boundaries, so that
items are grouped with their leading comments whenever possible when
chunking.

Finally, we've changed the `debug project index` action so that it opens
a simple debug view in a pane, instead of printing paths to the console.
This lets you click into a path and see how it was chunked.

Release Notes:

- N/A

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Co-authored-by: Marshall <marshall@zed.dev>