Add utility to create file based caches/indexes in editor

[HolonProduction]

While developing on godotengine/godot#99318 I really missed an easy way to implement a cache or index to quickly access some details about certain files. Implementing something like this from scratch for every PR that could benefit from it is just too tedious and error prone.

Also we have performance problems with GDScript autocompletion which could be solved with a longterm cache. But implementing it from scratch would just duplicate code, logic and bugs from other systems like the documentation cache.

I have worked a bit with the IntelliJ platform sdk recently and really like their approach to that. They offer a FileBasedIndex class were all you need to do is extract information from a file, and the IDE will handle updating the index based on file changes.

I think Godot could benefit from something similar as well. You would basically have an index class which you can register with the editor file system, which will then call some callbacks to update the index. Data could be accessed through the instance registered with editor file system.

Possible advantages I see:

• unified updating behavior should reduce the risk of out of date data in some systems
• could be used to facilitate a longterm GDScript cache and thus solve standing performance issues with autocompletion
• could be useful to editor plugins (both builtin and user made)

Possible problems:

• might not scale well with many caches (IntelliJ serializes indexes to not keep them in memory all of the time. This isn't really a thing in Godot AFAIK)
• Godot works largely cache based at the moment. While the IntelliJ system works index based (guaranteeing correct and update data in the index). A system based on recalculation when something changes might require multi threading to work properly. But some unified invalidation behavior would still be helpful IMO.