Fold length operations when argument is a constant string

Change is very self-explanatory, it seems a little odd this wasn't already added so if there is a reason that this doesn't already exist please say 👍

Compiler/src/Compiler.cpp

@@ -2892,10 +2892,13 @@ struct Compiler
                     result.valueNumber = -arg.valueNumber;
                 }
                 break;
-
             case AstExprUnary::Len:
+                if (arg.type == Constant::Type_String)
+                {
+                    result.type = Constant::Type_Number;
+                    result.valueNumber = double(arg.valueString.size);
+                }
                 break;
-
             default:
                 LUAU_ASSERT(!"Unexpected unary operation");
             }