Fix fmt on/off when multiple exist in leaf prefix (#1086)

The old behavior would detect the existence of a `# fmt: on` in a leaf
node's comment prefix and immediately mark the node as formatting-on,
even if a subsequent `# fmt: off` in the same comment prefix would turn
it back off. This change modifies that logic to track the state through
the entire prefix and take the final state.

Note that this does not fully solve on/off behavior, since any _comment_
lines between the off/on are still formatted. We may need to add
virtual leaf nodes to truly solve that. I will leave that for a separate
commit/PR.

Fixes #1005