default warning sets

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Currently each(?) rule has a configurable warnings default, e.g. @ocaml//executable:warnings=-1-2-3

Default is set to @[email protected]@30..39@[email protected]@[email protected]

Question: is that a good default? Should the rules even set a default?

One option is to define several 'warning sets' as config setting rules, so users can select by meaningful label. E.g. @ocaml//warnings:default set to the above, @ocaml//warnings:foo set to some other list, etc.

Insofar as warnings form semantic groupings, we could have one label per group (concept), and then we could compose warning sets.