Improve return type inferrability of in for homogeneous tuples

[aviatesk]

Overload _in_tuple for Tuple{Vararg{T}} so that, when the element type T is known not to intersect Missing, the return type is pinned as Bool rather than Union{Bool,Missing}.

This is motivated by JET(LS), which reports diagnostics when a non-Bool value appears in a branch condition. Without this change, straightforward code like:

function get_valid_flags(xs::Vector{String})
    flags = tuple(xs...)
    "flag" in flags ? flags : nothing  # non-boolean `Missing` found in boolean context (1/2 union split)
end

because the inferred return type of
::String in ::Tuple{Vararg{String}} is currently Union{Bool,Missing}, even though elements are all String and it's trivially clear that it never returns missing.

Implementation notes:

• Rather than adding a new in method for Tuple{Vararg{T}}, this overloads _in_tuple instead. Adding a separate ::Any in ::Tuple{Vararg{T}} method would increase the number of methods matching x::Any in itr::Tuple from 3 to 4, potentially degrading inferrability at abstract call sites.
• The additional hasintersect(T, Missing) would no runtime cost: the added _in_tuple is only compiled for concrete T, and in that case the typeintersect result will be statically inlined.