Block aware search

[heyman]

The current search feature is using the implementation from Codemirro which is not aware of blocks. Therefore it'll match the block delimiter lines (∞∞∞javascript).

Heynote should have a search feature that is "block aware" that wouldn't match the hidden delimiters, and where you can choose to only search the current block.

I believe this is best implemented as it's own feature, rather than trying to extend the Codemirror search (though we could copy relevant parts into Heynote if beneficial).

I also think that we can come up with an improved UI for the search feature.