adding serialize_precision's value that uses mode2 and long double

Hi, internals.

I had incorrect knowledge the serialize_precision until recently.
I knew that setting this to -1 would use a long double, but I didn't know that the number of digits would not be 17.

I've looked through RFC related to this.
https://wiki.php.net/rfc/precise_float_value

There doesn't seem to be a setting that allows you to use long double and mode2 at the same time at the moment.

I think there are various methods, such as a simple method such as simply setting serialize_precision = -2, or a method of adding another new setting item.

When using long doubles, it is somewhat inconvenient not to be able to display all digits with var_dump() or debug_zval_dump() when debugging.
(I use the word "inconvenient" because there is probably an alternative method.)

What do you think?

Saki