GDScript: Fix typing of lambda functions

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

@@ -0,0 +1,4 @@
+func test():
+	var lambda := func() -> int:
+		print('no return')
+	lambda.call()

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

@@ -0,0 +1,2 @@
+GDTEST_ANALYZER_ERROR
+Not all code paths return a value.

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

@@ -0,0 +1,4 @@
+func test():
+	var lambda := func() -> int:
+		return 'string'
+	print(lambda.call())

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

@@ -0,0 +1,2 @@
+GDTEST_ANALYZER_ERROR
+Cannot return value of type "String" because the function return type is "int".