make Base.ANSIIterator have a concrete field (#56088)

Avoids the invalidation

```
   backedges: 1: superseding sizeof(s::AbstractString) @ Base strings/basic.jl:177 with MethodInstance for sizeof(::AbstractString) (75 children)
```

shown in
#56080 (comment).

Co-authored-by: KristofferC <kristoffer.carlsson@juliacomputing.com>

Author: KristofferC

base/show.jl

@@ -72,13 +72,13 @@ ncodeunits(c::ANSIDelimiter) = ncodeunits(c.del)
 textwidth(::ANSIDelimiter) = 0
 
 # An iterator similar to `pairs(::String)` but whose values are Char or ANSIDelimiter
-struct ANSIIterator
-    captures::RegexMatchIterator
+struct ANSIIterator{S}
+    captures::RegexMatchIterator{S}
 end
 ANSIIterator(s::AbstractString) = ANSIIterator(eachmatch(ansi_regex, s))
 
-IteratorSize(::Type{ANSIIterator}) = SizeUnknown()
-eltype(::Type{ANSIIterator}) = Pair{Int, Union{Char,ANSIDelimiter}}
+IteratorSize(::Type{<:ANSIIterator}) = SizeUnknown()
+eltype(::Type{<:ANSIIterator}) = Pair{Int, Union{Char,ANSIDelimiter}}
 function iterate(I::ANSIIterator, (i, m_st)=(1, iterate(I.captures)))
     m_st === nothing && return nothing
     m, (j, new_m_st) = m_st