This commit checks in (but does not export) a prototype implementation of
Angular Signals, along with its unit test suite and a README explaining the
algorithms used.

Signals are not a new concept in the framework space, but there are many
different flavors of implementations. These differ radically both in terms
of public API as well as behavioral details (such as eager vs lazy
computation, batching behavior, equality, cleanup, nesting, etc).

This commit comprises a bespoke implementation that we've designed to best
meet Angular's needs, especially when it comes to compatibility and
flexibility of use within existing applications.

Many of the API features of this implementation of signals, as well as the
larger direction of reactivity in Angular, will be discussed in future RFCs.

Co-Authored-By: Pawel Kozlowski <pkozlowski.opensource@gmail.com>