Prioritize edited / recently viewed files for file explorer and search in the debugger

File explorer:
(internally) add edited files to the top of the libraries view with a subheader and a separator between those files and the rest
(externally) to do this, we'd have to have access to the IDE or git. feasible?

Search:
(internally) prioritize edited files in search results
(externally) prioritize recently viewed files (in history picker) in search results

Breaking this issue off into its own tracking issue from https://github.com/flutter/devtools/issues/3075