Add support for psr/log 2.0 and 3.0

[Teddytrombone]

Fixes #60
There are some caveats:

• as Psr\Log\Test\TestLogger was removed with psr/log:3.0 we need an alternative TestLogger for unit testing. colinodell/psr-testlogger provides this, which works for all psr/log versions.
• we need to ignore phpstan errors in bin/serve.php as the inline class inheriting AbstractLogger needs more type hints depending on the psr/log version

Apart from that everything should work. phpunit and phpstan ran without errors.