Give elements their own interface definition (#8290)

From what I understand, if we want to allow registering additional element options (see [here][1] for an example), then the element options need to be a top-level interface so that they can be used with TypeScript's [declaration merging][2].

[1]: https://github.com/chartjs/chartjs-plugin-annotation/pull/275#issuecomment-742754562
[2]: https://www.typescriptlang.org/docs/handbook/declaration-merging.html