Conditional Formatting in extLst

[oleibman]

Fix #4629. Excel Xml can specify Conditional Formatting in 2 ways - either using a conditionalFormatting tag as a child of worksheet, or in a slightly different manner as a child of worksheet->extLst. Although these are both handled by PhpSpreadsheet, the former is lost if there exists a latter whose cell range (sqref) matches it. This PR merges the former with the latter in that circumstance, preserving both.

The issue also notes that font and stopIfTrue are not handled correctly. Support for those was missing in the extlst handling, and is now added with this PR.

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