Added separate Eval mode with option to isolate the environment

This allows for keeping variables isolated

ucl/builtins/os_test.go

@@ -28,7 +28,7 @@ func TestOS_Env(t *testing.T) {
 			inst := ucl.New(
 				ucl.WithModule(builtins.OS()),
 			)
-			res, err := inst.Eval(context.Background(), tt.eval)
+			res, err := inst.EvalString(context.Background(), tt.eval)
 			assert.NoError(t, err)
 			assert.Equal(t, tt.want, res)
 		})