Save all shortcuts to `keymap.json`, not only overrides


thread-list:mark-all-as-read nor composer:send-message (as well as some others) aren’t in either of the following keymap files in the repo:

I only learnt about it when I defined a shortcut for the action.

I think that saving all keyboard shortcuts to keymap.json is much better, with disabled shorcuts set to an empty string (thus the shorcut is disabled even now).

When a new keyboard command is added to Mailspring, it should be added to keymap.json.