Add TypeScript types for observer parameters. (#5359)
Add TypeScript types for observer parameters. (#5359)

- PolymerDeepPropertyChange for .* observers.
- PolymerSpliceChange and PolymerSplice for .splices observers.
- Parameterized for the property/path types.
- Matches the name of the interfaces in polymer-externs.js.

Polymer/polymer,Polymer/polymer,Polymer/polymer