Revised and modernized

This release updates the code in the following ways:

1. Type hints all parameters and returns, now passes PHPStan level 6.
2. Moves PHP version to 7.4.
3. Updates development tooling to use modern self-contained PHPUnit and PHPStan.
4. Replaces require: icecave\isolator with require-dev: php-mock/php-mock-phpunit to remove the PHP global function calls with mocks only during testing, instead of having the proxy running in the library at all times.