Implement Search

[justinpombrio]

Four levels of search:

• General. Search highlighting. A Search is a predicate on nodes. Highlight any node that matches the predicate. n/N jumps to the next/prev node that matches the predicate (via next/prev_inorder). There's a shortcut to clear the search.
• General. Make overlapping search & cursor highlighting not awful.
• 1. Jump to next/prev node of given type
• 2. Find exact matches of the current node (like * in Vim). Should highlight and count the matches, ideally being lazy if there are a lot.
• 3. Substring search, over all texty nodes.
• 4. Tree search with "." for any single node and ".*" for zero or more nodes in a listy context.

MVP is at least (1), probably (1-3).

[e-matteson]

1 is done in 7fb5e37