This adds a non-intrusive mechanism for indicating that an API is
expected to be deprecated "soon" where "soon" means at least the
next semver-major Node.js release. The `DeprecateSoonWarning` is
only emitted if the `--deprecate-soon` command line option is
used which means the option will have zero impact for most
situations. For those developers who wish to be proactively
alerted to pending deprecations, use of the `--deprecate-soon`
option can be used in testing/CI environments to determine
which APIs may end up deprecated in the future.

It should be noted that a "deprecate soon" warning is just a
heads up, and that a decision can be made later NOT to
deprecate the API if the change is deemed to be too
intrusive.