Customize handling of un-bridgable types/methods

[marcprux]

When a public type or method cannot be bridged from Swift to Kotlin, the skipstone plugin will fail with an error. For packages that might have a large portion of public API that isn't expected it be bridgable, it can be cumbersome to annotate these all with // SKIP @nobridge.

We could have a skip.yml option called unbridgable that accepts fail (the default), warn, and ignore. The latter two would avoid attempting to create a bridge to a type or method that isn't bridgeable.

[piercifani]

For what is worth, I think it could be simpler to have an option in the .yml to either:

• Bridge everything public
• Opt into bridging using an annotation like // SKIP @bridge