fix: add import of .css file into another .css file

After changing `css-loader` with `css2json-loader`, we have an issue that module imports of local css files doesn't work anymore - #1098. As it turns out that we don't have applications that test this scenario, we decided to add such an import in demo JavaScript application.

demo/JavaScriptApp/app/app.android.css

@@ -10,6 +10,7 @@ of writing your own CSS rules. For a full list of class names in the theme
 refer to http://docs.nativescript.org/ui/theme.
 */
 @import '~nativescript-theme-core/css/core.light.css';
+@import 'app.common.css';
 
 ActionBar {
  background-color: #7F9;

demo/JavaScriptApp/app/app.ios.css

@@ -10,6 +10,7 @@ of writing your own CSS rules. For a full list of class names in the theme
 refer to http://docs.nativescript.org/ui/theme.
 */
 @import '~nativescript-theme-core/css/core.light.css';
+@import 'app.common.css';
 
 ActionBar {
  background-color: #999;