Confusing `levels` fallback

Looking at the concrete implementation of levels in CategoricalArrays, I see that this function conveniently extracts possible levels from both arrays and individual values:

julia> a = CategoricalArray(["abc", "def", "abc"])
3-element CategoricalArray{String,1,UInt32}:
 "abc"
 "def"
 "abc"

julia> x = a[1]
CategoricalValue{String, UInt32} "abc"

julia> levels(a)
2-element Vector{String}:
 "abc"
 "def"

julia> levels(x)
2-element Vector{String}:
 "abc"
 "def"

However, the fallback implemented in DataAPI itself is only correct for collections, not individual values:

# like unique(x), makes sense?
julia> levels(["abc", "def", "abc"])
2-element Vector{String}:
 "abc"
 "def"

# makes no sense:
julia> levels("abc")
3-element Vector{Char}:
 'a': ASCII/Unicode U+0061 (category Ll: Letter, lowercase)
 'b': ASCII/Unicode U+0062 (category Ll: Letter, lowercase)
 'c': ASCII/Unicode U+0063 (category Ll: Letter, lowercase)

# especially confusing given that:
julia> x == "abc"
true

Maybe, the fallback shouldn't exist at all, and something like haslevels(x)::Bool added instead?

Start with the DataAPI levels fallback and the Julia examples in this issue. Decide the intended behavior for individual values versus collections, including whether a haslevels API is needed, then define focused tests for the chosen contract; no specific file or test is named.