Add option to handle use of features marked for deprecation as errors

[lukastaegert]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:
Resolves #2945

Description

Previously the procedure when deprecating a feature in Rollup was this

• Mark it as deprecated in the types
• In the next major version, add a warning that the feature is deprecated
• In the major version after that, remove the feature

The problem with this approach is that there is no way for e.g. plugin developers to get a notification about upcoming deprecated features before the next major version is released. Also, deprecation warnings are easily overlooked.

To that end, a new option strictDeprecations is added. When this flag is set to true

• using a deprecated feature will throw an error
• upcoming deprecated features that do not show a warning yet will also throw errors when used

My recommendation is for plugin authors to use this flag e.g. as part of their CI workflow.