Convert template_preprocess in views

This might not be as hard as I feared, but it does depend on this central piece of magic:

       $initial_preprocess_class = 'Drupal\\' . $def['provider'] . '\\Hook\\' . Container::camelize($def['provider']) . 'ThemeHooks';
        $initial_preprocess_method = 'preprocess' . Container::camelize($def['theme']);

        if (method_exists($initial_preprocess_class, $initial_preprocess_method)) {
          $hooks[$def['theme']]['initial preprocess'] = $initial_preprocess_class . ':' . $initial_preprocess_method;
        }

I think we generally want less magic like this (service providers), but it's not enforced, just a default, and it feels in line with the kind of magic that this hook does.

There is also the bit about theme_file that plugins can in theory provide, but the new attribute classes are actually missing that and I want to deprecate the underlying 'file' key as well, so I just went ahead and added that deprecation right now. There are a bunch of them in contrib, but again, they would need to resort to a plugin definition alter hook for this in D12 with attributes anyway. And all they need to do for D12 compatibility is move the functions into their .module file (or convert to initial preprocess, but that would require 11.3 unless we backport this)

I now also created the second CR about theme_file (which, as mentioned in the CR, is basically already deprecated in D12 as we forgot to add it to the attribute classes and rarely used, so I thought it's OK to explicitly deprecate for D12).

FWIW, I'm not entirely happy about the approach that I implemented, writing the CR and explaining it reminded me of that again. Too much magic. An alternative is putting it on on the specific plugin class, that would be a much better location, but I don't really see a way to make that work with DI, it would need to be a static method. The ThemeHooks class approach has the advantage that DI works. But the whole feature is pretty magical already and entirely optional. You can alter it, you can also not define a theme key and fully define it yourself.

Rebased, only had two conflicts on test classes, some use and docblocks in core/modules/views/tests/src/Kernel/ViewsPreprocessTest.php and core/modules/views/tests/src/Kernel/Handler/ArgumentSummaryTest.php. Should be fine to set back to RTBC assuming that test are green.