Add support for static variables in GDScript

Which allows editable data associated with a particular class instead of
the instance. Scripts with static variables are kept in memory
indefinitely unless the `@static_unload` annotation is used or the
`static_unload()` method is called on the GDScript.

If the custom function `_static_init()` exists it will be called when
the class is loaded, after the static variables are set.

modules/gdscript/tests/scripts/analyzer/errors/static_constructor_with_return_type.gd

@@ -0,0 +1,5 @@
+static func _static_init() -> int:
+	print("static init")
+
+func test():
+	print("done")

modules/gdscript/tests/scripts/analyzer/errors/static_constructor_with_return_type.out

@@ -0,0 +1,2 @@
+GDTEST_ANALYZER_ERROR
+Static constructor cannot have an explicit return type.

modules/gdscript/tests/scripts/analyzer/errors/static_var_init_non_static_call.gd

@@ -0,0 +1,9 @@
+@static_unload
+
+func non_static():
+	return "non static"
+
+static var static_var = non_static()
+
+func test():
+	print("does not run")

modules/gdscript/tests/scripts/analyzer/errors/static_var_init_non_static_call.out

@@ -0,0 +1,2 @@
+GDTEST_ANALYZER_ERROR
+Cannot call non-static function "non_static()" for static variable initializer.