make `@syms` support functions with a variable number of arguments

[matthias314]

In Symbolics.jl one can say @variables f(..) to define a function that accepts a variable number of arguments. In SymbolicUtils.jl the analogous macro call using @syms defines a function with a single argument:

julia> typeof(@syms f(..))
Tuple{SymbolicUtils.Sym{SymbolicUtils.FnType{Tuple{Number}, Number}, Nothing}}

One can get a function with a variable number of arguments by saying something like

f = SymbolicUtils.Sym{SymbolicUtils.FnType{Tuple, Number}}(:f)

It would be great if one could use @syms f(..) instead.