feature: only recompute dirty docs during editing

[benjamincburns]

Problem

When editing docs w/ hot reload enabled, it appears that on each refresh all blocks from all docs recompute.

Describe the solution you'd like

It'd be handy if I could make it so that only the docs that have changes recomputed. Ideally it'd also only recompute the blocks that changed, but that would require pickling the execution content and storing it somewhere, and I'm not sure that we could do that for JS/TS execution once #70 merges.

[benjamincburns]

Ah, I think this may duplicate #4 ?

[pawamoy]

Yeah I think we can close this one in favor of #4, which should handle the feature requested here.