CODE/UNICODE and CHAR/UNICHAR

[oleibman]

PhpSpreadsheet treats CODE and UNICODE as equivalent, likewise for CHAR and UNICHAR. They are, in fact, different. CODE and CHAR deal only with single-byte character sets (Windows-1252 or MacRoman), while UNICODE and UNICHAR deal with all of Unicode. This PR separates them. The existing unit test for CODE was, in many cases, applicable to UNICODE (for which there was no separate test). The tests are corrected for CODE, new tests are added, and a separate test for UNICODE is added. CHAR was mostly okay, new tests are added, and a separate test for UNICHAR is added.

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