TOML: Improve type-stability (#55016)

Dependent on #55017. This changes
the output of the TOML parser to provide specialized `Vector{T}` less
aggressively.

Specifically, combinatorially expensive types like
`Vector{Vector{Float64}}` or `Vector{Union{Float64,Int64}}` are instead
returned as `Vector{Any}`, while vectors of homogeneous leaf types (e.g.
`Vector{Float64}`) are still supported as before.

This change makes the TOML parser fully type-stable.