Add annotate! method for AnnotatedIOBuffer (#53284)
The annotate! function provides a convenient way of adding annotations
to an AnnotatedString/AnnotatedChar without accessing any of the
implementation details of the Annotated* types.
When AnnotatedIOBuffer was introduced, an appropriate annotations method
was added, but annotate! was missed. To correct that, we refactor the
current annotate! method for AnnotatedString to a more general helper
function _annotate! that operates on the annotation vector itself, and
use this new helper method to easily provide an annotate! method for
AnnotatedIOBuffer.
