Tweak SyntaxList constructor to accept tree elements (#61133)
It should be easy to create known-length lists of trees without repeated
`push!`ing, but it isn't, and desugaring is full of
`x = SyntaxList(ctx); push!(x, st1); push!(x, st2)`.
This change adds a `SyntaxList(::SyntaxTree...)` method. Previously, we had
`SyntaxList(::SyntaxTree)` create an empty list in
that tree's graph (but that method was mostly unused).
This also matches other utilities where we'll often have one function
taking SyntaxGraph+NodeId(s) and one taking SyntaxTree(s).
We could also consider using `Vector{SyntaxTree}` instead (I'm not sure
how much performance benefit there is to `SyntaxList` in the first place), though
that would require some refactoring of all the existing stuff that takes SyntaxList.
