fix: allow vector defaults in @mtkmodel

[ven-k]

Closes #2810

These work now:

   @mtkmodel VectorDefaultWithMetadata begin
        @parameters begin
            n[1:3] = [1, 2, 3], [description = "Vector defaults"]
        end
    end

   @mtkmodel VectorConditionalDefault begin
        @structural_parameters begin
           flag = true
        end
        @parameters begin
            n[1:3] = if flag
                [2, 2, 2]
            else
               1
            end
        end
    end

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API