actions: Enable new manifest workflow

The manifest workflow uses the manifest action to detect changes in the
west manifest. It then analyzes the changes and posts labels and a
comment in table format accordingly.

It is meant to be used as a helper bot for developers submitting changes
to modules, reducing the need for manual work and oversight and
automating common operations.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>