Limit the number of repeats for commands which create or remove tabs

Adds an optional `repeatLimit` property for commands. If the command is called with a `count` higher than this limit (when specified), we prompt to confirm that the user wants to repeat the command this 'large' number of times.

This property has been set on all commands which create or remove tabs. Addresses #1126.

Also disables numerical prefixes to Vomnibar commands, which don't work at present.
This does work, at least for me. It uses [`window.confirm`](https://developer.mozilla.org/en-US/docs/Web/API/Window.confirm), a browser built in.