* Added TagHelperFeature and TagHelperFeatureProvider to perform tag helper discovery.
* Changed tag helper discovery to use application parts when using tag helpers as services.
* Added FeatureTagHelperTypeResolver to resolve tag helper type definitions from the list of application parts.
* Added AddTagHelpersAsServices extension method on IMvcBuilder and IMvcCoreBuilder that
performs tag helper discovery through the ApplicationPartManager and registers those tag helpers as
services in the service collection. Assemblies should be added to the ApplicationPartManager
in order to discover tag helpers in them in them. The @addTagHelper directive is still required on
Razor pages to indicate what tag helpers to use.
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