feat: retain IO metadata in toexpr(::AbstractSystem)

Author: AayushSabharwal

src/systems/abstractsystem.jl

@@ -1822,6 +1822,17 @@ function push_vars!(stmt, name, typ, vars)
             ex = nameof(s)
         end
         push!(vars_expr.args, ex)
+
+        meta_kvps = Expr[]
+        if isinput(s)
+            push!(meta_kvps, :(input = true))
+        end
+        if isoutput(s)
+            push!(meta_kvps, :(output = true))
+        end
+        if !isempty(meta_kvps)
+            push!(vars_expr.args, Expr(:vect, meta_kvps...))
+        end
     end
     push!(stmt, :($name = $collect($vars_expr)))
     return