Add @return $this annotation to methods

[lorenzolosa]

#3777 and #3779 added generics to InteractsWithMedia and FileAdder, which improves PHPStan recognition of return types. However, PHPStan is still unable to follow through chaining; for instance, it will interpret the return value of:

$this->addMedia($file)
    ->usingFileName($filename)
    ->toMediaCollection('collection');

as the base Media class, even if an extended class has been specified.

This PR fixes that by listing the return type of a bunch of methods as $this. These include methods in the FileAdder class, needed for the example above; but I have also included several others (mostly with generics, where I believe it's most useful, but not only).

[freekmurze]

Thanks!