Fixes #421

Demo: https://insert-connection.blockly-keyboard-experimentation.pages.dev/

Problems with this PR:

• Select create canvas block, insert statement block, escape to abort. We delete more than we intend.
• Not doesn't wrap around true in test of if by default if on if

To raise as separate issues:

• Post-abort cursor position isn't great (like delete in general).
• Undo has too many steps.