Add reverse search functionality

[AloeareV]

It looks a good deal less efficient than forward search, but searching a 1500+ line file for 'a' doesn't seem to have caused any noticeable latency for me.

I wasn't sure what to do with extend_search_next and extend_search_prev, given that there's a shortage of letters left available, so I bound them to H and L.

This is my first commit that's actually adding/renaming functions, let me know if there are any naming conventions I should be aware of.

[archseer]

The note in keymap.md should be removed too https://github.com/helix-editor/helix/blob/master/book/src/keymap.md#search

I think it's okay to use a suboptimal implementation (forward search is really inefficient too since we have to turn the Rope into a String) and we'll replace it once regex-automata supports NFA simulation.

[archseer]

Great work! 🎉