Add a `canAnnotate` method to the `Integration` interface to indicate
whether a range is part of the annotatable content of the document. This
allows integrations to signal whether the Annotate/Highlight toolbar
should be shown for a given selection.

Implement it for PDFs by returning `true` if the range is part of the
text layer of a single page. Implement it for HTML documents by always
returning true.

Fixes #3790