[context] Type definitions aren't consistent.

It looks like I missed updating some of the type declarations in the protocol description in https://github.com/webcomponents-cg/community-protocols/pull/36 This has caused some [interop problems](https://github.com/lit/lit/issues/4609) with implementations that used the previous object-based contexts that we decided to change in https://github.com/webcomponents-cg/community-protocols/issues/19.