Add Spreadsheet Method for Duplicating Worksheet

[oleibman]

Cloning a worksheet attached to a spreadsheet creates a clone which is detached from the spreadsheet. This can have its uses, but I think it would also be useful to have the ability to duplicate the worksheet and keep the duplicate attached to the spreadsheet. You can do that in Excel and LibreOffice, and you can now do it in PhpSpreadsheet as well. The duplicated worksheet will come immediately after its source.

The worksheet being duplicated could be identified in a number of ways - by passing the worksheet itself to the new method, by passing the worksheet title, or by passing the index of the worksheet within the spreadsheet. For now, I am just implementing the one I think is most useful (title).

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?

Provide an explanation of why this change is needed, with links to any Issues (if appropriate).
If this is a bugfix or a new feature, and there are no existing Issues, then please also create an issue that will make it easier to track progress with this PR.