Change window.plugins type to interface

Change the type of window.plugins from an object literal specific to Push.d.ts to an interface called Plugins, which can be modified by other code.
Rename module, directory and filename to follow CONTRIBUTING.md
Merge branch 'master' of github.com:iyegoroff/DefinitelyTyped

# By Keisuke Oohashi (1) and Michael Kourlas (1)
# Via Masahiro Wakame (2) and Michael Kourlas (1)
* 'master' of github.com:iyegoroff/DefinitelyTyped:
  Change window.plugins type to interface
  Rename module, directory and filename to follow CONTRIBUTING.md