Replace the "plugin" functionality in the Guest that indirectly
instantiates the `DocumentMeta`, `PDF` and `CrossFrame` classes with
direct instantiation of those classes.

The annotator part of the Hypothesis client is no longer a pluggable
toolbox which can be arbitrarily extended by user-provided plugins.
Removing the "plugins" logic and replacing it with direct construction
of the necessary classes removes a lot of indirection that made the code
harder to follow and also makes it easier for TypeScript to statically
check the code. It will also enable the `config` object that is passed
to `Guest` to be paired down to just the fields that are actually needed
and properly typed.

In future we will need some kind of plugin/integration facility to
support different document viewers. For we will build a more specific
document type integration facility when the time comes.

 - Remove `this.plugins` field from `Guest` class and `pluginClasses`
   config and replace with direct construction of `PDFIntegration`,
   `DocumentMeta` and `CrossFrame` classes

 - Update the `AnchoringImpl` typedef to correctly reflect that some anchoring
   implementations (HTML) have a synchronous `describe` method

 - Remove unnecessary `config` argument to `PDF` constructor

 - Replace the `PDF: {}` content in the guest config with `documentType:
   'pdf'` as a more obvious way for the annotator entry point to
   configure which document viewer integration is loaded. In future we
   can generalize this to support different types of viewer integration.