#### Description
[13864](https://dev.azure.com/fluidframework/internal/_workitems/edit/13864/)
This PR adds forkable revertible feature to the `Revertible` object of
SharedTree.
- Removed `DisposableRevertible` and replaced by `RevertibleAlpha`.
- Added `clone()` method to the new interface.
- Uses `TreeBranch` (which is subset of `TreeCheckout`) to access data
necessary for revert operation.
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Co-authored-by: Noah Encke <78610362+noencke@users.noreply.github.com>
Co-authored-by: Joshua Smithrud <54606601+Josmithr@users.noreply.github.com>
Co-authored-by: Jenn <jennle@microsoft.com>
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