`extra` keybindings should have precedence over `base`

[CosmicHorrorDev]

The purpose of extra was to give the user control over changing just a couple of keybindings here and there while still making use of the defaults (base). The current design partially fulfills this purpose by giving the option to add additional keybindings, but it's limited in that you can't override individual keybindings. It would be nice if extra keybindings had a higher precedence than base in that keybindings that appear in both will use the extra version instead of choking on a conflict

Notably this won't address conflicts where a keybinding in one prefixes some keybinding in the other, but I don't know of a good resolution there. I think as it stands now any additional keybindings will have to be added with a semver breaking bump