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/parser/errors/static_constructor_not_static.gd

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

modules/gdscript/tests/scripts/parser/errors/static_constructor_not_static.out

@@ -0,0 +1,2 @@
+GDTEST_PARSER_ERROR
+Static constructor must be declared static.

modules/gdscript/tests/scripts/parser/errors/static_constructor_returning_something.gd

@@ -0,0 +1,6 @@
+static func _static_init():
+	print("static init")
+	return true
+
+func test():
+	print("done")

modules/gdscript/tests/scripts/parser/errors/static_constructor_returning_something.out

@@ -0,0 +1,2 @@
+GDTEST_PARSER_ERROR
+Constructor cannot return a value.