Exclude `silverstripe-theme` modules from the report

[robbieaverill]

As previously discussed with @be2n and @clarkepaul we will not be showing themes or recipes in the report

[robbieaverill]

We need to replace this logic with something extensible:

return Package::get()->filter('Type:StartsWith', 'silverstripe');

In the composer update checker module we have an "allowed_types" config flag which defines the "type" (in composer.json) options that are allowed to be included. We could centralise that into this module (maybe UpdatePackageInfo.allowed_types config prop) and then replace the use in the update checker module too.