Add throws tags and better generics

[defunctl]

Main Changes

Due to the missing throws tags, developers don't actually know that most of these method calls will throw an exception from the stellarwp/db library.

I THINK I got all the places this could happen, but due to the __callStatic in the db library, the IDE doesn't really know.

• Additionally, this uses generics to type hint where we'd take a static DB class, which lets PHPStan perform some better static analysis and give your IDE autocompletion.
• Adds WP CLI stubs for PHPStan
• tests are failing because I believe the config has to be migrated to the new WpBrowser version, for now just install wp-browser versions less than 3.5
• Updates GitHub action versions to their latest.

[dpanta94]

Looks good!

[lucatume]

The changes look great, thank you.

When it comes to testing, the update of wp-browser can be done in another PR.