GDScript: Allow classes declaration to be done in single line

Incidentally, allow multiple statements in single line functions when
using semicolon as a terminator.

modules/gdscript/tests/scripts/parser/features/single_line_declaration.gd

@@ -0,0 +1,11 @@
+#GDTEST_OK
+
+func test(): C.new().test("Ok"); test2()
+
+func test2(): print("Ok 2")
+
+class A: pass
+
+class B extends RefCounted: pass
+
+class C extends RefCounted: func test(x): print(x)