Firefox starts element picking by sending pick to the walker actor. Add
matching protocol handling so Ladybird can enter picker mode, send
hover, preview, pick, and cancel events, and serialize picked nodes in
the shape Firefox expects.
Picker hit testing can return text nodes, but Firefox only selects
elements in this mode. Normalize picker events to the closest element
ancestor and include the parent chain so Firefox can materialize missing
tree nodes before selecting the target.
97070f836c