Use Core/Scene stringnames consistently

[KoBeWi]

Follow-up to #81303
Replaces #50941

This PR solidifies usage of CoreStringNames and SceneStringNames by replacing every string instance by its singleton equivalent, if it already existed. I also removed some duplicate or unused names. I might have missed some very common names, as it's difficult to find them in all replaceable contexts.

Up to discussion, but I included core string names in variant.h and scene string names in node.h and resource.h. This makes them easily accessible without extra includes. These files are not modified often, so it's probably fine to do that. I removed all other includes of these files. I didn't do the same for EditorStringNames, because they are included in much more files and there is no single file that's included in all editor code.

Note that it's rather unreasonable to enforce using singleton string names when available,. We could just replace some newly added strings from time to time. Maybe I'll make a helper script for that.

[KoBeWi]

Ok I don't know where to put scene string names include :/
Maybe it could stay in node.h, so at least some files won't need it. But otherwise there is no way to avoid including it everywhere.

[AThousandShips]

Were planning to do one of these myself, and looks great!

[akien-mga]

Didn't review in depth, but makes sense to me.

[KoBeWi]

I wrote some simple Ruby script that scans the codebase for StringNames:
https://pastebin.com/ZaYn57YP
It can detect unused or underused singleton StringNames, duplicate names and suggest replacement for raw strings. Though it needs smarter filters/exceptions, because right now it spams over 2k Strings 🤔

EDIT:
This PR seems to have reduced the spam by 700.

[akien-mga]

Thanks!