Eagerly parse all files in the project after project load.

Once https://github.com/dotnet/fsharp/issues/14848 is fixed and FCS has a usable parsing cache,
can we add optional eager parsing of all files upon project load?
The end goal is to reach a point where every parsing request except one for the currently edited file is hitting the cache.

- Parsing is quicker than type-checking, so much less potential for putting IDE to a crawl.
- Ideally configurable by end-user, so no drawbacks for users who don't want to use it.
- Solution should take into account cache size and either stop eager parsing if the cache is overfilling or increase it when a large codebase is loaded.
- Parsing requests should be asynchronous and not block the UI in any meaningful way.

Related:
"Too many parsing requests" issue: https://github.com/JetBrains/resharper-fsharp/issues/492

Start by reviewing the prerequisite FCS issue 14848 and the related “Too many parsing requests” issue. Done would mean an optional, asynchronous eager parse after project load that respects parsing-cache size and does not meaningfully block the UI.

Written by the indexing model from the issue text.