Linting - API

[nedtwigg]

• This is a replacement PR for #1097

Spotless is still primarily a formatter, not a linter. But when formatting fails, it's more flexible to model those failures as lints so that the formatting can continue. For example, issues like

• #287
• #655

are difficult to resolve with our current model, because the only channel we have to communicate with the user is "this is the formatted content". Now we have two channels: "this is the formatted content" and "these are the problems we had".

At this point, the purpose of these lints is just to do formatting better. They are an implementation detail. If people start submitting PRs for more linters, perhaps it will turn into something more. Who knows!