YaruMasterDetailThemeData: implement ThemeExtension interface

[jpnurmi]

Implementing the ThemeExtension interface for YaruMasterDetailThemeData makes it possible to use it in (Yaru)ThemeData.extensions.

YaruTheme(
  data: const YaruThemeData(
    extensions: [
      YaruMasterDetailThemeData(
        ...
      )
    ],
  ),
  builder: (context, yaru, child) {
    return MaterialApp(
      theme: yaru.theme,
      darkTheme: yaru.darkTheme,
      home: Example.create(context),
    );
  },
)