Modal editing

This is what Vim does at its core. You can switch mode, and in different modes, the editor behaves differently. This can be powerful, but may not be beginner friendly.

Whereas Vim has few visual clues about what can be done and what can't in Insert/Visual/Select modes, I believe there is room for a user-friendly use modes. For example, we can imagine a "Displace" mode, where one can select and move blocks around with the keyboard, but not type. Pressing ESC would exit the mode, back into "Natural" mode.

We will not consider this for now, for simplicity. But this could become relevant when implementing a "rich diff" mode.