Registers are stored inside `Editor` and accessed without `RwLock`.
To work around ownership, I added a sister method to `Editor::current`:
`Editor::current_with_context`. I tried to modify `Editor::current`
directly but it's used at a lot of places so I reverted into this for
now at least.
User can select register to yank into with the " command.
A new state is added to `Editor` and `commands::Context` structs.
This state is managed by leveraging a new struct `RegisterSelection`.
