.editorconfig: Set charset for all files to UTF-8

Closes GH-620.

Enhancement: Trim trailing whitespace in all files

Closes GH-633.

Enhancement: Add Makefile

Closes GH-621.

Test UserNotes\Sorter

We add a copy of the latest run-tests.php from the PHP-8.0 branch, and
tests for the `UserNotes\Sorter` with full code coverage. We also run tests
on GitHub Ac

Test UserNotes\Sorter

We add a copy of the latest run-tests.php from the PHP-8.0 branch, and
tests for the `UserNotes\Sorter` with full code coverage. We also run tests
on GitHub Actions.

Co-authored-by: Andreas Möller <am@localheinz.com>

Closes GH-606.

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Enhancement: Add .editorconfig

Closes GH-554.