Strip _xlfn. and _xlws. In Formula Translations

[oleibman]

Fix #3819. Excel can add these prefixes (basically invisible to end-user). Formula translation in PhpSpreadsheet fails when dealing with these unexpected prefixes, and, even if it handled it correctly, the unexpected prefixes confuse the users. I have changed to strip those prefixes when translating to a locale. This is probably not perfect, but is almost certainly good enough. I could easily add the same change when translating from a locale to English, but I don't think there's a good use case for that, so am opting not to do so for now.

The documentation mentions translateFormulaToLocale and translateFormulaToEnglish. Neither of these exist; both names are preceded by an underscore. I have changed the code to match the documentation rather than vice versa, retaining deprecated versions of the underscored routines which merely invoke the non-underscored routines.

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 this change is needed?

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