R1:1aa8bc319b52

Search symbols by ctrl/cmd-click

Summary:
Refs T8302.
V1 of the implementation. This replaces the previous mode, but I guess there's no real reason we can't have
some symbols always clickable and the rest require modifier.

I'm also a little concerned about discoverability; Holding down ctrl/cmd will make the cursor change, so there's
some hint that something might be up, but that's probably not obvious enough.

Test Plan:
Tested in diffusion and differential and differential comments on:
- Windows/Chrome…