This is technically imprecise, but this widening is useful for two
reasons:
  1. It prevents the compiler from creating `AnnotatedString` types "out
     of thin air", such as when doing `join(expr.args...)` which is
     inferred as `::Union{AbstractString,String}`
  2. It is close to what inference will be able to determine if we try
     to make `*` / `join` adopt `promote_type`-like behavior in general,
     rather just as a special case for AnnotatedString (like it is now).

(1) is particularly important because the out-of-thin-air behavior can lead
to the compiler inspecting AnnotatedString methods that aren't available
until StyledStrings loads (and therefore invalidate)

An obvious (and quite common) case of this is:
  `print(io, join(v..., " "))`
which w/o this change is a bad invalidater