Feature: Support PHP 8 and add non-blocking I/O support on Windows with PHP 8. (#85 by @clue)

Minor documentation improvements. (#78 by @WyriHaximus and #80 by @gdejong)

Improve test suite and add .gitattributes to exclude dev files from exports. Run tests on PHPUnit 9, switch to GitHub actions and clean up test suite. (#75 by @reedy, #81 by @gdejong, #82 by @SimonFrings and #84 by @clue)