Add disclosable component (#2868)

This PR adds a disclosable component, related wiring, and uses it to
implement the collaboration panel's disclosure of subchannels. It also
adds a component test page to make style development easier, and
refactors components into v0.2, safe styles (as described in [TWAZ
#16](https://zed.dev/blog/this-week-at-zed-16))

Release Notes:

- N/A