Typed `this` inside JS patches

Commit message

[IMP] web: type `this` inside patch extensions

Annotate the `extension` parameter with `ThisType<T & U>` so that `this`
inside a patch resolves to the patched object (target plus extension)
rather than to the extension literal alone. Patch authors then get
completion and type checking on the target's members.

`super` cannot be covered the same way: TypeScript always resolves
`super` in an object literal method to `any`, whatever the contextual
type, so no annotation can reach it. Typing it would mean passing the
original as a named value, i.e. changing the `patch` API and every call
site, which is not worth it here.