Add migration to add `<force>` to `mode.normal.esc`

Add migration to add `<force>` to `mode.normal.esc`

We already migrate old shortcut customizations to the new 0.6.0 format. However,
I forgot that if the user had customized the shortcut for the esc command, it
most likely needs `<force>` at the beginning as well.

This commit adds `<force>` to every shortcut for that command (if needed).
Ideally this should have been done already in the 0.6.0 release. Now we run the
tiny risk of adding `<force>` to a shortcut that the user actually intended not
to have `<force>` in, but it's worth it: See #577, #571, #582 and #583.