Add support for _xlfn. prefixed functions + implement ISFORMULA

[Josh-G]

This is:

- [ ] a bugfix
- [x] a new feature

Checklist:

• Changes are covered by unit tests
• 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
• Documentation is updated as necessary

Why this change is needed?

This change adds support for newer functions that are prefixed by _xlfn. (#356). The calculation engine has been updated to recognise these as functions, and drop the _xlfn. part. I've added a couple of the new functions such as STDEV.S/P, MODE.SNGL, ISFORMULA.

Whilst testing ISFORMULA, I see that loading and saving a spreadsheet that contains true or false values, without making any modifications causes the spreadsheet to contain integer 0 or 1 when loaded again in Excel. Could the following code block be added here

PhpSpreadsheet/src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php

Line 1058 in 0084776

elseif (is_bool($calculatedValue)) {
    $objWriter->writeAttribute('t', 'b');
}

to resolve this?

Let me know if my PR needs amending.

[PowerKiKi]

That looks very good. We should only figure out what to do when $pCell is null and then it should be mergeable.

[PowerKiKi]

Either $pCell should not be optional or there should be a check whether it is null and do something else instead. Maybe add something like:

        if (!is_object($pCell)) {
            return Functions::REF();
        }

and cover it with tests.

[Josh-G]

Thanks for the review. I wasn't sure under what circumstances it could be null? I've updated it to return a REF error instead and modified the test.

Any thoughts on the boolean save conversion I mentioned above?

[PowerKiKi]

Thank you for this nice contribution.

For the boolean save conversion I suggest to create a new PR and it could be covered with functionnal tests, probably something similar to tests/PhpSpreadsheetTests/Functional/FreezePaneTest.php.