Support built-in scripts for inspector categories

[KoBeWi]

This is sort of follow-up to #32428 (which seems completely untested with built-in scripts btw).

It adds better support for built-in scripts in the inspector:

• Most notably, it fixes Editor crash when selecting a node with a built-in script #40117
• Built-in script category is now named "Built-in script"
  
• Fixed loading built-in script class icon resulting in error (by not trying to load it bruh)

Notes:

• (script->get_path() != String() && script->get_path().find("::") == -1)
  This condition is a result of a bug (probably). Built-in scripts seem to have empty path until the scene is saved, each time the scene is reloaded. The condition could be changed when that issue is resolved.
• ERR_FAIL_COND(!s->is_valid());
  This prevents editor crash in case the script fails to load (could happen e.g. when the file is removed externally, perhabs).

[aaronfranke]

I'm going to be honest, I completely forgot that built-in scripts existed when I gave that a ✔️

[akien-mga]

Thanks!

[willnationsdev]

Ah, crap, yeah, I should've caught that. My bad. I haven't checked GitHub in a little while, so that'd be why I didn't fix it earlier. Glad it got fixed quickly-ish.