Add extension contexts for Calva states such as project root, session type, ns, etcetera

[PEZ]

Sometimes the most effective way to automate something is to create a keybinding. But keybindings are global, while they may not make sense outside a given project context.

Keybindings can have when clauses, and these can refer to when clause contexts including extension contexts. We already have a few of these (like calva:connected). If we added a few more, and the right ones, we would enable better targeted keybindings. Then something like ctrl+alt+r r could do different things in different projects/contexts.