Text validation

[justinpombrio]

E.g.: if you put q in a number, that edit should fail.

Approach:

• Every texty construct has a validation regex.
• PrettyDoc.node_style() highlights a texty node in red if its validation regex fails.
• Exit text command fails with a message if the text is invalid, and reverts if repeated. The message can say something like "Text is invalid. Repeat exit text command to revert your changes."
• Empty text is still special, and needs to survive a save/load cycle (though perhaps converting into a hole).