Direct copy relevant files to platform_interface.

This commit contains a pure copy of the `lib/platform_interface.dart`
and `lib/src/webview_method_channel.dart` files from the
`webview_flutter` package to the `webview_flutter_platform_interface`
package.

The `auto_media_playback_policy.dart` and `javascript_mode.dart` contain
the `AutoMediaPlaybackPolicy` and `JavascriptMode` enums copied directly
from the `webview_flutter/lib/webview_flutter.dart` package. The code
doesn't contain any modifications.

Author: mvanbeusekom