feat(developer): hint on unreachable code

Fixes #355.
Fixes #1710.

This is structured as a feature so it is tested as such, but was part of
the set of fixes scheduled for 15.0 beta. Any rule in a group that
Keyman Developer assesses as 'unreachable' will receive a hint message.

The current rule match is fairly simplistic -- it only does an exact
comparison -- so duplication which is indirect, through `any(store)`
references in the context would still result in unreachable rules that
are not picked up.