Remove zend-stdlib dependency

[Trainmaster]

Description

Fixes #1256

Checklist:

• I have run composer run-script check --timeout=0 and no errors were reported
• The new code is covered by unit tests (check build/coverage for coverage report)
• I have update the documentation to describe the changes

[troosan]

@Trainmaster a better solution would be to use PhpOffice\Common\Text:: isUTF8 instead.
I'll see if can un-deprecate it (@Progi1984?).

[Progi1984]

👌 with that.

[Trainmaster]

@troosan I changed it to PhpOffice\Common\Text:: isUTF8.

[JamesB7]

I think the is_string check is important: preg_match will return true for non-strings, such as integers. (Yes, this surprised me too. Hurrah for PHP?)

Without the is_string check, callers of isUTF8 (such as toUTF8) may pass through non-string values without converting them to string (such as utf8_encode does).

[troosan]

I create a PR in PHPOffice/Common#19 to modify the method

[Progi1984]

Merged ;)