Update web implementation to use platform interface.

just_audio_web/pubspec.yaml

@@ -0,0 +1,25 @@
+name: just_audio_web
+description: Web platform implementation of just_audio
+homepage: https://github.com/ryanheise/just_audio/tree/master/just_audio_web
+version: 0.0.1
+
+flutter:
+  plugin:
+    platforms:
+      web:
+        pluginClass: JustAudioPlugin
+        fileName: just_audio_web.dart
+
+dependencies:
+  #just_audio_platform_interface: ^0.0.1
+  just_audio_platform_interface:
+    path: ../just_audio_platform_interface
+  flutter:
+    sdk: flutter
+  flutter_web_plugins:
+    sdk: flutter
+  meta: ^1.1.8
+
+environment:
+  sdk: ">=2.7.0 <3.0.0"
+  flutter: ">=1.12.13+hotfix.5"