common/assert: Add UNIMPLEMENTED_IF and UNIMPLEMENTED_IF_MSG for conditional assertions

Currently, there's no way to specify if an assertion should
conditionally occur due to unimplemented behavior. This is useful when
something is only partially implemented (e.g. due to ongoing RE work).
In particular, this would be useful within the graphics code.

The rationale behind this is it allows a dev to disable unimplemented
feature assertions (which can occur in an unrelated work area), while
still enabling regular assertions, which act as behavior guards for
conditions or states which must not occur. Previously, the only way a
dev could temporarily disable asserts, was to disable the regular
assertion macros, which has the downside of also disabling, well, the
regular assertions which hold more sanitizing value, as opposed to
unimplemented feature assertions.