Make capabilities check be enforced by type system

[pokey]

So eg setHighlights could be null or something, and so that way you have to check. Maybe have type descriminator field and in one version its null and in another it's not. It's possible that we would just want to add a commands field to editor that contains all these to make it easier to make typing work

• [ ] Make it so we can easily document which capabilities actually could result in improved behaviour via cursorless fallback if they're undefined. Maybe use enum? That might make this type-based linking harder though

[pokey]

see also https://github.com/cursorless-dev/cursorless/issues/1160