LibJS: Return early from parseFloat() if argument is a number

This saves us both a bit of time and accuracy, as Serenity's strtod()
still is a little bit off sometimes - and stringifying the result and
parsing it again just increases that offset.

Libraries/LibJS/Runtime/GlobalObject.cpp

@@ -153,6 +153,8 @@ Value GlobalObject::is_finite(Interpreter& interpreter)
 
 Value GlobalObject::parse_float(Interpreter& interpreter)
 {
+    if (interpreter.argument(0).is_number())
+        return interpreter.argument(0);
     auto string = interpreter.argument(0).to_string(interpreter);
     if (interpreter.exception())
         return {};