Add filters to the addon AddonServiceProvider.php

[jonassiewertsen]

I did notice that we explicitly do allow to register tags, fieldtypes, modifiers, widgets and others, but no filters.

This PR does add the possibility to register filters like other components: https://statamic.dev/extending/addons#registering-components

protected $filters = [
    \Acme\Example\Filters\First::class,
    \Acme\Example\Filters\Second::class,
    // etc...
];

I am happy to update the docs as well, if this is getting merged.

[jasonvarga]

Nice, but can we call these scopes instead?

[jonassiewertsen]

Should we add actions as well?

[jasonvarga]

Sure

[jasonvarga]

Thanks!