Glean-parser

- A new parser config, `do_not_disable_expired`, was added to turn off the
feature that expired metrics are automatically disabled. This is useful if you
want to retain the disabled value that is explicitly in the `metrics.yaml`
file.
- `glinter` will now report about superfluous `no_lint` entries.

- A `glinter` tool is now included to find common mistakes in metric naming
and setup. This check is run during `translate` and warnings will be
displayed. ⚠ These warnings will be treated as errors in a future revision.

- BUGFIX: `GleanGeckoMetricsMapping` must include `LabeledMetricType`
and `CounterMetricType`.

- NEW: Support for outputting metrics in Swift.
- BUGFIX: Provides a helpful error message when `geckoview_datapoint` is used on
an metric type that doesn't support GeckoView exfiltration.
- Generate a lookup table for Gecko categorical histograms in
`GleanGeckoMetricsMapping`.
- Introduce a 'Swift' output generator.

- Documentation only.

- Added support for generating markdown documentation from `metrics.yaml` files.