REPL: Convert char to string before calling escape_string

[DilumAluthge]

In this function, key is allowed to be either String or Char, but there is no method matching escape_string(::Char). So convert key to a string before calling escape_string.

MWE:

import REPL

keymap = Dict{Char, Any}()
REPL.LineEdit.add_nested_key!(keymap, 'a', "foo")
REPL.LineEdit.add_nested_key!(keymap, 'a', "bar")

Which throws this error:

julia> REPL.LineEdit.add_nested_key!(keymap, 'a', "bar")
ERROR: MethodError: no method matching escape_string(::Char)
The function `escape_string` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  escape_string(::IO, ::Union{Base.AnnotatedString, SubString{<:Base.AnnotatedString}}, ::Any; keep, ascii, fullhex)
   @ Base strings/annotated_io.jl:266
  escape_string(::IO, ::Union{Base.AnnotatedString, SubString{<:Base.AnnotatedString}}; ...)
   @ Base strings/annotated_io.jl:266
  escape_string(::AbstractString, ::Any; keep, ascii, fullhex)
   @ Base strings/io.jl:467
  ...

Stacktrace:
 [1] add_nested_key!(keymap::Dict{Char, Any}, key::Union{Char, String}, value::Any; override::Bool)
   @ REPL.LineEdit ~/juliaup.depot/juliaup/julia-1.12.5+0.aarch64.apple.darwin14/Julia-1.12.app/Contents/Resources/julia/share/julia/stdlib/v1.12/REPL/src/LineEdit.jl:1785
 [2] add_nested_key!(keymap::Dict{Char, Any}, key::Union{Char, String}, value::Any)
   @ REPL.LineEdit ~/juliaup.depot/juliaup/julia-1.12.5+0.aarch64.apple.darwin14/Julia-1.12.app/Contents/Resources/julia/share/julia/stdlib/v1.12/REPL/src/LineEdit.jl:1779
 [3] top-level scope
   @ REPL[3]:1

But we want it to throw this instead:

ERROR: Conflicting definitions for keyseq a within one keymap

---

Detected by JET.