GDScript: Disallow return with value in void functions

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

@@ -0,0 +1,2 @@
+func test() -> void:
+  return null

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

@@ -0,0 +1,2 @@
+GDTEST_ANALYZER_ERROR
+A void function cannot return a value.

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

@@ -0,0 +1,4 @@
+func test() -> void:
+  var a
+  a = 1
+  return a

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

@@ -0,0 +1,2 @@
+GDTEST_ANALYZER_ERROR
+A void function cannot return a value.

modules/gdscript/tests/scripts/analyzer/features/return_variant_typed.gd

@@ -0,0 +1,5 @@
+func variant() -> Variant:
+  return 'variant'
+
+func test():
+  print(variant())

modules/gdscript/tests/scripts/analyzer/features/return_variant_typed.out

@@ -0,0 +1,2 @@
+GDTEST_OK
+variant