RFC: Add Form Feed in Trim Functions

[LamentXU123]

I searched keyword trim in the codebase and fix every case I think related to the three functions. I didn't touch other features with trim implemented and missing \f. (such as trim in php filters or other code)
This PR changes:

• The default parameters characters of trim (also ltrim and rtrim) in the php interface, adding \f, source
• The behavior in php_trim_int (which effects php_trim), source
• Test case concerning trim, adding case of \f.

I am sending mails to internals now. After that I will write the NEWS file (and docs) if this feature is accepted.
doc of trim: https://www.php.net/manual/en/function.trim.php
This is my first time contributing to php so I am not so sure about this PR. Thanks.

ps: also a little detail. I change the default parameter to \f\n\t\r\v\0 using alphabetic orders. (abcdefghijklmn...)

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RFC: https://wiki.php.net/rfc/trim_form_feed
Fixes GH-20783

[LamentXU123]

RFC (https://wiki.php.net/rfc/trim_form_feed) implemented, with specific test case.
I am convinced it is ready to be reviewed.

[TimWolla]

The default value specified in ext/standard/basic_functions.stub.php also needs to be adjusted.

[TimWolla]

The implementation itself LGTM now.

[TimWolla]

@LamentXU123 Thank you. Now merged. Don't forget to move your RFC into the “Implemented” section on the overview. Within the RFC document you should update the status as well and link to the implementation commit: da1e89f

[LamentXU123]

All done. Thank you.
Oh also I think the manual page of those trim functions should be updated. Is there a need for me to open another PR?