Partial Support of Fill Handles

[oleibman]

Fix #3847. Person posing the question calls this behavior "pull formulas", and wants equivalent handling to be available in PhpSpreadsheet. I can do this for formulas in a single cell; I can go horizontal or vertical or (unlike Excel) both. Nevertheless, I call this "partial support" because I cannot think how I can do something similar that Excel does, e.g. putting 2 in cell A1 and 4 in Cell A2, selecting them both and using the fill handle to extend those to rows below, so that cell A5 will contain 10.

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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