OS should be highlighted as a class

[atirut-w]

Godot version

v3.5.stable.arch_linux

VS Code version

1.71.2

Godot Tools VS Code extension version

1.3.1

System information

N/A

Problem statement

OS is highlighted similarly to func, var, etc. which seems to be incorrect even according to Godot's script editor:

Proposed solution

Change the highlight color to be the same as other singletons.

[DaelonSuzuka]

Already fixed in #402.