This mode is activated through the new key-bindings.unicode-input and
search-bindings.unicode-input key bindings.
When active, the user can “build” a Unicode codepoint by typing its
hexadecimal value.
Note that there’s no visual feedback in this mode. This is
intentional. This mode is intended to be a fallback for users that
don’t use an IME.
Closes#1116
