Restore options namespace for controller options (options[type].* being too weak)

[simonbrunel]

I was looking at the new options structure in v3 and I realize that we now have options[type].* with type the controller type (if I'm not wrong). But type could be anything such as flow, control, function, tree, interaction, events, etc.: we don't know because anyone can create a new chart type and use whatever name.

And that's a big issue because that means we can't guarantee no breaking change every time we add or rename a root options (e.g. core options). For example, if someone creates an external chart using type 'foo' and then we decide to introduce a new foo core option, then we break this external project for absolutely no good reason. It's also a shame to have to tell Chart.js users to not pick any name from our core options as a chart type.

That's for the same reasons we decided to scope the plugins options.

I would suggest to revert #6955 (and probably rename options.datasets[type] to options.controllers[type] to be consistent with Chart.defaults.controllers[type]).

[kurkle]

I went ahead and added the blocker status, because I agree we don't want to be stuck with potentially breaking external projects every time a new root option is added.