Convert JumpToBlock to a list of multi-key bindings

JumpToBlock is special: each of its 5 elements is the binding for one side
window (status / files / branches / commits / stash), not an alternate for a
single command. Change the field from []string to []Keybinding so each window
slot can have alternates of its own.

The schema becomes "an array of 5 keybindings, each itself a string or array of
strings", which falls out cleanly from how the Keybinding type inlines into the
generated schema. Existing configs (a flat array of 5 strings) keep validating
because each element is unmarshalled through Keybinding's scalar-or-sequence
decoder.