State for the describe-key meta-mode.
When active, describe-key intercepts all input to walk the keymap
trie and report the binding at the terminal node. The three fields
(leader_node, keys) always change together: entering describe-key
sets leader_node to the trie root, and exiting (via match, escape,
or unbound key) resets both fields by replacing the struct with nil.
Summary
Types
@type t() :: %Minga.Mode.DescribeKey{ keys: [String.t()], leader_node: Minga.Keymap.Bindings.node_t() | nil }