Fix type stability of `ReadEachIterator` (#56678)
Its `eltype` method had an incorrect type declaration. The preexisting
tests did not detect this because they check for array equality, where
`Any[] == Char[]` is satisfied.
### Before
```julia-repl
julia> x = readeach(IOBuffer(), Char);
julia> eltype(x)
Any
julia> collect(x)
Any[]
```
### After
```julia-repl
julia> x = readeach(IOBuffer(), Char);
julia> eltype(x)
Char
julia> collect(x)
Char[]
```
---------
Co-authored-by: Jameson Nash <vtjnash@gmail.com>
Co-authored-by: Ian Butterworth <i.r.butterworth@gmail.com>
Co-authored-by: Dilum Aluthge <dilum@aluthge.com>
Co-authored-by: Andy Dienes <51664769+adienes@users.noreply.github.com>
