Slightly Better Support for Escaped Characters in Xlsx Reader/Writer

[oleibman]

See Discussion 4724

PhpSpreadsheet converts all control characters (x00-x1f) in strings to and from a form which Excel recognizes (e.g. x1c becomes _x001C_ when writing, and vice versa when reading). There have historically been 3 exceptions which go unconverted - tab (x09), line feed (new line) (x0a), and carriage return (x0d). PR #4536 removed those exceptions, but that caused some problems; these were fixed by PR #4619, but the exceptions were restored.

The referenced discussion deals with a spreadsheet with a cell containing _x000D_, carriage return. Although the writer no longer converts to that string on output, the reader should be able to handle it on input. In fact, the reader ought to handle any string of the form "underscore x 4-hex-digits underscore", whether or not it represents a control character.

And there's an interesting edge case. If a user enters into a cell the string A_x0030_B, it needs to be handled as-is. Excel handles this by writing it out as A_x005F_x0030_B, i.e. substituting _x005F_ for the first underscore, so that the reader sees _x005F_ (converting it to underscore) followed by x0030_B (no leading underscore, so no conversion). PhpSpreadsheet could probably handle this by converting all underscores on write, but I am trying to emulate Excel and do it only when needed.

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