[core] optimize and improve decorations

[vince-fugnitto]

Description

Based on the #6132 (comment)

The rendering of decorations throughout the application can be improved and optimized in order to reduce the total amount of renders required while consequently improving performance.
At the moment, decorations are quite costly as there are unecessary rendering present.

One such costly decoration are decorations contributed by problems, @akosyakov had an idea:

I had an idea to keep in ProblemManager:

• a mapping between uri -> stat (count of warnings, errors, infos)
• and total stat
  So when a change event happens we update a single stat by uri and then update a total stat by subtracting previous uri stat and adding new uri stat. So we can update problem stat in the status bar very fast, maybe even without any debouncing like now. It would require testing with many diagnostics though again, i.e. as in Dart extension support PR.

If we would also add an event like onDidStatChanged(uri), then we could change ProblemTabBarDecorator based on uri stats instead of changes to markers directly and decorate title also very fast by retrieving a uri stat and checking how many errors or warnings it has (so traversing happens only once to compute stat for a uri, not by each client on each request).