Proposal: Improve feature deprecation procedure

[lukastaegert]

Expected Behavior / Situation

At the moment, deprecating a feature works the following way:

• TS types are updated immediately
• In the next major (I know we only had one so far, this is how I currently intend to do it), using the feature will produce a warning, ideally with a hint what to do instead
• In the major after that, the feature is removed

Actual Behavior / Situation

Other than watching for type changes, there is little that aspiring (plugin) developers can do to get notified of upcoming deprecations. And even once a feature has been deprecated, warnings are easily overlooked.

Modification Proposal

I propose to add a new option strictDeprecations that will

• throw errors instead of warnings for deprecations
• throw also for upcoming deprecations

The intention is for (plugin) developers to use it e.g. in their CI to get early feedback about upcoming changes. This flag could also be used internally by Rollup to keep tests up to date and explicitly mark tests for deprecated features.