Make Vararg not a DataType (#38136)
Currently `Vararg` is a DataType, but is special cased in a
bunch of places to give it special behavior (e.g. in subtyping
and of course in tuple construction). However, unlike all other
DataTypes, it cannot appear as a type parameter, which caused trouble in
PR #38071. Having it be a DataType is a bit of a pun of convenience
in the first place - it's a lot more similar to a tvar (which can
be considered an implementation detail of UnionAll in the same way
Vararg is an implementation detail of Tuple), which has its own
non-type object. This PR does the same to Vararg, and moves it
from being an abstract DataType with special cased behavior to
its own custom type (called `Core.TypeofVararg`). There are a
few small behavior differences, but they are mostly internal.
In particular, we no longer have `Vararg <: Type` and Vararg
objects no longer have the .parameters field. Also, things like
`Vararg{T} where T` are technically illegal now since Vararg is
not a type. However, since a lot of people are using that pattern,
I've brought it back with a deprecation (which is of course off
by default). The only things that's disallowed is `Vararg{N, N} where N`,
but I haven't seen anybody use that.
