Add PHP 8.3 support, drop PHP < 8.0

• Drop support for PHP < 8.0
• Adds type hints and return types etc
• Updates a few libraries

See also #6

github build improvements

• build on github actions for PHP '7.2', '7.3', '7.4', '8.0', '8.1' & '8.2'
• psalm/phpdoc fixes