TOML: Return Dates objects by default, even for explicit Parser

[topolarity]

The doc-comment here strongly suggests this was intended to be a public interface (although not the most frequently used one), so this tries a bit harder to avoid breaking the Dates-related behavior here.

This removes DTParser, which effectively behaved the way the existing Parser() behaved before #54755

As a side effect, this hides the Base.TOML.Parser type out of the way the public API.

[KristofferC]

Will it be confusing if TOML.Parser() doesn't return a TOML.Parser. Say:

struct Cache
    parser::TOML.Parser
    ....
end

I wonder if the only way to do this "properly" is to have two Parser structs, one in Base, one in TOML, both <: AbstractTOMLParser and compile the toml parsing code once for each of them.

[topolarity]

OK, I think I have a plan that'll work:

1. Add the Parser{Dates} type parameter in this change, but don't delete the .Dates field (to avoid breaking Pkg). TOML.Parser() will return a Parser{Dates} which isa TOML.Parser
2. Change Use Base.TOML.Parser{Dates} for TOML parsing w/ Dates support Pkg.jl#3938 to use Base.TOML.Parser{Dates}()
3. Once Pkg is updated, remove the .Dates field in TOML: Improve type-stability #55016

Let's handle (1) in #55017 and I'll close this PR for now.