Code editor improvement: highlight singletons

[Zylann]

I just discovered that it's possible to use auto-load singletons by their names, as if they were global variables.

PlayerSettings.load()
# Instead of this
get_node("/root/PlayerSettings").load()

It would be nice to have them coloured in the code editor because otherwise they look like variables coming from nowhere. Usually, you can look up for the definition of a member or class by searching the file, but if it's a global thing you don't know that by just looking at the code.

[Calinou]

Bugsquad note: This issue still seems to be relevant as of b030834.

[Zylann]

Nowadays, something adds up to the intent behind that issue: named classes are also global-scope, maybe it makes sense to highlight them too?