Add read/write specialisation for IOContext{AnnIO}

Ensure that when an AnnotatedIOBuffer is wrapped in an IOContext, that
writes of annotated strings/chars and reading out an AnnotatedString is
unimpeded by the IOContext wrapping.

Without these specialisations, the generic pipe_reader/pipe_writer
fallbacks will directly access the underlying IOBuffer and annotations
will be lost.

There are a number of scenarios in which one might want to combine an
AnnotatedIOBuffer and IOContext (for example setting the compact
property). Losing annotations in such scenarios is highly undesirable.

The read specialisation isn't actually needed to avoid this headache,
but seems nice to have for completeness.

base/show.jl

@@ -444,6 +444,12 @@ function show_circular(io::IOContext, @nospecialize(x))
     return false
 end
 
+# Specialisations that would be put in `annotated.jl`, but for the fact it's loaded first.
+write(io::IOContext{AnnotatedIOBuffer}, s::Union{AnnotatedString, SubString{<:AnnotatedString}}) =
+    write(io.io, s)
+write(io::AnnotatedIOBuffer, c::AnnotatedChar) = write(io.io, c)
+read(io::IOContext{AnnotatedIOBuffer}, T::Type) = read(io.io, T)
+
 """
     show([io::IO = stdout], x)