Proposal: New simpler onwarn API

[Conduitry]

As discussed in chat, it may be nice to simplify the onwarn callback API. Currently, it accepts two arguments, a warning object and the default onwarn handler. Consumers can inspect the warning object and decide whether and how to call the default onwarn handler.

Instead, it could just accept a warning object, and return a boolean, indicating whether the compiler should display this warning. Slightly less powerful (but I don't see any real uses for the power we would use), and simpler to understand.

[Conduitry]

Another radical, even simpler idea: No onwarn handling in the core compiler at all. It just returns the array of warnings in the stats, and it's up to the bundler plugin or whatever to have some way to handle it. This might be a bad idea. But there is something appealing to me about making the compiler a completely pure function - no side-effects, no i/o of any sort.

[Rich-Harris]

I like the second idea, it makes a lot of sense

[Rich-Harris]

closed via #2093