Violation of the (informal) `Number` interface

I have been hitting my head at #330 for quite some time now, and I just realized that it's due to the type TaylorN claiming to be a <: Number, but deviates from the interface for getindex for numbers:

julia> x, = set_variables("x")
1-element Vector{TaylorN{Float64}}:
  1.0 x + 𝒪(‖x‖²¹)

julia> x
 1.0 x + 𝒪(‖x‖²¹)

julia> x isa Number
true

julia> x === x[]
true

julia> x === x[1]
false

For any scalar x::Number, the operations x[] and x[1] are expected to be identical, but for TaylorN they aren't.

Reproduce the Julia example with TaylorN and compare x[] with x[1]. Start by locating TaylorN's getindex implementation and the Number interface behavior; done means both forms return the same scalar value while preserving existing indexing behavior. Verify the result with focused TaylorN tests or the project's test suite.