chore(context): Expose `elgg_*_context` functions as a service

TODO:
- [x] Expose the `context` service in our service provider
- [x] Update the `@since` annotations
- [x] Fill out tests, includes tests for the global wrappers
- [x] Rename `get` to `peek` since that's what it does
- [ ] Incorporate Elgg\Context service (https://github.com/Elgg/Elgg/pull/7101)
- [ ] Switch to namespaces (https://github.com/Elgg/Elgg/pull/7104)
Let's hope devs weren't relying on this