Add @template-covariant support

[arnaud-lb]

No description provided.

[JanTvrdik]

That seems way too long. Why not extend the existing @template annotation to support in/out keywords (Kotlin) or +/- signs (Scala)?

/**
 * @template in T
 */

[ondrejmirtes]

We need to support what's already in the wild. But we also plan to support @phpstan-generic<K, V> syntax and prefer it in the docs etc. See the discussion there (and feel free to contribute to it!): phpstan/phpstan#2527

[muglug]

That seems way too long. Why not extend the existing @template annotation to support in/out keywords (Kotlin) or +/- signs (Scala)?

The reason I went with -covariant is that it seemed to me, as a newcomer to the idea, something that should be annotated fairly explicitly.

Also I felt out was confusing given by-ref parameters: vimeo/psalm#1603 (comment)