Add namespaces (#121)

* Add namespaces

This adds the concept of namespaces, as discussed in whatwg/console#3 (starting especially around whatwg/console#3 (comment)). They can only contain regular operations.

The ES binding for namespaces here is written in a fully modern style, and so differs slightly from similar prose for interfaces' ES binding. It is hoped that it can provide a template for eventually updating interfaces' ES binding to modern ES.

* Address most code review comments:

- Missing some [SecureContext] stuff
- Not yet addressed the operation-creation stuff

* Stick with "declared on" instead of "declared within"

It doesn't read quite right to me, but let's keep it as-is for now and investigate any issues in #153.

* Move "creating an operation function" out to #es-operations