Updated to newer phpstan

[lsv]

I was thinking on how I should structure a slack office project, and while I did that, I did some human work :)

• Updated to phpstan ^1.0 (still php 7.4 combatible)
• Updated to a more aggresive phpstan level 7
• Updated over 200 @param, @var @return etc. phpdocs
• Removed composer.lock - As this is a library, you should not have a composer.lock
• Added github action to also test matrix, php 7.4, 8.0 and 8.1

If I get more time, I will properly also up the coverage% - But that will be in a new PR

[jeremeamia]

Thanks! I'm in the middle of a move, but I'll review everything as soon as I can. I appreciate the contributions.

[lkck24]

How about merging this PR?