[Shaders] Disallow trailing commas in function declarations

[AThousandShips]

Might not be the most elegant fix, but not very familiar with this code, if someone has a cleaner idea for doing this I'd appreciate suggestions. Added it with deprecation to avoid breaking existing shaders, though I'm not sure that's necessary

I was considering enclosing the whole while loop in a block to check closing paren at the start, which would make catching this easier, but want to avoid noise in the blame if I can, having to indent the whole thing.

For reasoning, see the discussion in:

• Shaders: Allow trailing commas in function calls godot-proposals#8840

Alternative to:

• Shaders: Allow trailing commas in function calls #86991

[AThousandShips]

Could add a warning on non-disabled cases (i.e. default builds) if desired

Tested and it doesn't look good and isn't noticeable, also feels like a bad idea to add a new warning category just for this to allow it to be configured well