GDScript: Allow strings as multiline comments

Bring back the behavior in 3.x that was left out by oversight.

modules/gdscript/tests/scripts/parser/warnings/standalone_expression.gd

@@ -1,6 +1,5 @@
 func test():
 	# The following statements should all be reported as standalone expressions:
-	"This is a standalone expression"
 	1234
 	0.0 + 0.0
 	Color(1, 1, 1)

modules/gdscript/tests/scripts/parser/warnings/standalone_expression.out

@@ -8,14 +8,10 @@ GDTEST_OK
 >> STANDALONE_EXPRESSION
 >> Standalone expression (the line has no effect).
 >> WARNING
->> Line: 5
+>> Line: 6
 >> STANDALONE_EXPRESSION
 >> Standalone expression (the line has no effect).
 >> WARNING
 >> Line: 7
 >> STANDALONE_EXPRESSION
 >> Standalone expression (the line has no effect).
->> WARNING
->> Line: 8
->> STANDALONE_EXPRESSION
->> Standalone expression (the line has no effect).