New reference panel: filter out defs on isExternalPrivateSymbol

Created by: mrnugget

Part of #30970 (closed). This replicates another bit from code-intel-extensions: the filtering by isExternalPrivateSymbol.

I started trying to port the existing filter but then realised it'll be easier if I copy the Result and SearchResult types from code-intel-extensions too. That's why the diff is so long. It's 95% copy&pasted code.

And it also fixes two TODOs from before.

Test plan

• Tested this manually to check that it doesn't break anything. But not sure how to replicate the exact behaviour that this filters out.

App preview:

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