RIGHT: return all of the text when num_chars exceeds its length

[kevin9327]

This is:

• a bugfix
• a new feature
• refactoring
• additional unit tests

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change and a link to the pull request if applicable
• Documentation is updated as necessary

Why is this change needed?

RIGHT drops characters from the start of the result when num_chars is bigger than the text:

=RIGHT("abc",5)   -> "bc"   (Excel: "abc")
=RIGHT("abc",4)   -> "c"    (Excel: "abc")

The start offset mb_strlen($value) - $chars goes negative, and mb_substr then counts it from the end of the string. Excel's docs say "If num_chars is greater than the length of text, RIGHT returns all of text", and LibreOffice does the same. This PR clamps the offset at 0.

I added two cases to the RIGHT data provider: one ASCII and one multibyte, where the length is exactly one more than the text. Both fail without the fix. The TextData function tests pass, and php-cs-fixer, phpcs, and phpstan are clean on the changed file.

[oleibman]

Thank you for your contribution.