Fix #4312. Excel applies Conditional Formatting rules according to a priority specified in the xml. The priority must be a natural number; the rules are applied in order from lowest priority number to highest. When reading an Xlsx spreadsheet, PhpSpreadsheet has been ignoring the priority, which can result in differences from Excel's behavior, especially when CF cell ranges overlap (note that overlapping ranges are not supported in Xls format).

If an application uses PhpSpreadsheet to add new Conditional Formatting to a worksheet and does not change its priority from the default (0), the Xlsx Writer will assign a priority with a higher value than any of the CF objects which have been assigned a priority (either from reading it or explicitly assigning it).

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.