Glean-parser

- The `glinter` command now performs `yamllint` validation on registry files.

- The Kotlin linter `detekt` is now run during CI, and for local
testing if installed.
- Python 3.8 is now tested in CI (in addition to Python 3.7). Using
`tox` for this doesn't work in modern versions of CircleCI, so the
`tox` configuration has been removed.
- `yamllint` has been added to test the YAML files on CI.
- ⚠ Metric types that don't yet have implementations in glean-core
have been removed. This includes `enumeration`, `rate`, `usage`, and
`use_counter`, as well as many labeled metrics that don't exist.

- Allow a Swift lint for generated code
- New lint: Restrict what metric can go into the `baseline` ping
- New lint: Warn for slight misspellings in ping names
- BUGFIX: change Labeled types labels from lists to sets.

- Use lists instead of sets in Labeled types labels to ensure that the order of
the labels passed to the `metrics.yaml` is kept.
- `glinter` will now check for duplicate labels and error if there are any.

- Add labels from Labeled types to the Extra column in the Markdown template.

- BUGFIX: Don't call `is_internal_metric` on `Ping` objects.