Unify `IDisposable` interfaces

[CraigMacomber]

Description

Unify IDisposable interfaces from @fluidframework/tree and @fluidframework/core-interfaces.

Breaking Changes

Public APIs in @fluidframework/tree now use IDisposable from @fluidframework/core-interfaces replacing disposeSymbol with "dispose".

IDisposable in @fluidframework/core-interfaces is now @sealed indicating that third parties should not implement it to reserve the ability for Fluid Framework to extend it to include Symbol.dispose as a future non-breaking change.

Reviewer Guidance

The review process is outlined on this wiki page.