Tell PHPStan which method returns which object

[ilicfilip]

Context

Earlier today we had a fatal PHP error, it was caused by a method being removed on a branch which was merged into develop. A bit later other branch, which used the removed method, was also merged into develop but the method in question was missed (quite easily since we had so many changes).

The problem is that PHP error was triggered in a very specific use case, when plugin was updated, so it wasn't obvious.

But we have tests in place, so how did PHPStan miss it?
Because we use magic methods to get object, so PHPStan doesn't know which object to expect when do \progress_planner()->get_suggested_tasks()->get_tasks_by_status( 'pending' );

This PR adds @method annotations which tell PHPStan which methods to expect.
Since we're close to release I haven't added annotations for all classes, some of them require PHPStan fixes and I don't want to do that a day before the release.

The objects which we use most often are covered, most importantly \Progress_Planner\Suggested_Tasks and PHPStan errors and warnings for it are fixed.

I also tried implementing a more elegant approach, by defining a Dynamic Return Type Extensions, but it consumed too much memory and it was pretty slow.

Quality assurance

• I have tested this code to the best of my abilities.
• I have added unit tests to verify the code works as intended.
• I have checked that the base branch is correctly set.

[github-actions]

Test merged PR on Playground
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