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.
Note that the style in the code base is to have spaces around `{}`. Unless there's a strong external convention not to change it, let's leave it as is. If we did ever decide to change it, we would want to do it for the whole code base in one commit, so we don't do it piecemeal in each PR.