Add fork() to SharedTree's Revertible Interface

[jikim-msft]

Work in Progress

Description

ADO 13864

This task adds fork() method to the Revertible interface which (as name suggests) forks Revertible object and grants separate control.

This would allow scenarios such as calling revert or disposed on forked revertible while NOT affecting the source branch.

Notes

• Changed the location of Revertible.ts from core to shared-tree directory for two reasons:

1. fork() method is expected to receive forked view (of type TreeCheckout) which is inaccessible in the core directory.
2. The documentation suggests that the Revertible interface is intended for SharedTree thus decided to move its location.