fix broken `annotate!` for `AnnotatedChar` (#60488)
The function `annotate!` has been broken for `AnnotatedChar`, at least
since the inclusion StyledStrings.jl into Julia 1.11:
```
julia> using StyledStrings; annotate!(AnnotatedChar('x'), :face, :red)
ERROR: FieldError: type NamedTuple has no field `value`, available fields: `label`, `val`
Stacktrace:
[1] macro expansion
@ ./namedtuple.jl:129 [inlined]
[2] @NamedTuple{label::Symbol, value}(nt::@NamedTuple{label::Symbol, val::Symbol})
@ Base ./namedtuple.jl:124
[3] annotate!(c::AnnotatedChar{Char}, label::Symbol, val::Any)
@ Base ./strings/annotated.jl:370
[4] top-level scope
@ REPL[1]:1
```
This PR fixed this:
```
julia> using StyledStrings; annotate!(AnnotatedChar('x'), :face, :red)
'x': ASCII/Unicode U+0078 (category Ll: Letter, lowercase)
```
(The red color is shown in the REPL.)
Side remark: Given that no-one can use this function at present, I
wonder if one should reconsider the choice of arguments. Even if the
annotations do not form a dictionary, I would see some benefit in
writing
```
annotate!(c, label => value)
```
An extension to more than one annotation
```
annotate!(c, label1 => value1, label2 => value2)
```
would then be more natural.
