Allow Replace of Dummy Function with Custom Function

[oleibman]

Custom functions were introduced recently, with a restriction that non-custom functions could not be replaced. However, reviewing a recent issue has led me to the conclusion that it might sometimes be impractical to implement an Excel function in PhpSpreadsheet for all users, but it could still be helpful to some users to offer an implementation anyhow.

The Excel ASC function is currently unimplemented in PhpSpreadsheet, and is a candidate for replacement using the functionality added in this PR. This PR supersedes PR #4513, which I will now close, and its earlier incarnation PR #4511. For reasons discussed in 4513, I don't see a way forward for implementing the Excel ASC function in a way that would be generally usable. However, this PR would permit a user to implement ASC in a way which would satisfy the user's local requirements. See the new testReplaceDummyFunction; I think, or at least hope, that, despite the deficiencies of 4511 and 4513, this might satisfy the author's requirement.

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

[oleibman]

Another day ending in 'y' means another false positive for Scrutinizer. It is now suppressed.