dartdoc directive handling is internally inconsistent and inefficient

[jcollins-g]

Some dartdoc directives are implemented via an ad-hoc plugin system with buildDocumentationAddition, others are implemented directly as part of DocumentationComment, and the whole thing kind of comes together a bit awkwardly.

Redesign this so that each directive implementation is split out into its own class, and DocumentationComment can have different directives registered as valid on different symbol types (Class, Library, etc).