Glean-parser

- Leverage the `glean_namespace` to provide correct import when building for Javascript.

- The Kotlin generator now generates static build information that can be passed
into `Glean.initialize` to avoid calling the package manager at runtime.

- Add support for generating Javascript code.
- The templates added generate metrics and pings code for Glean.js.

- New versions 2.0.0 of the `metrics.yaml` and `pings.yaml` schemas now ship
with `glean_parser`. These schemas are different from version 1.0.0 in the
following ways:

- Bugs must be specified as URLs. Bug numbers are disallowed.
- The legacy ping names containing underscores are no longer allowed. These
included `deletion_request`, `bookmarks_sync`, `history_sync`,
`session_end`, `all_pings`, `glean_*`). In these cases, the `_` should be
replaced with `-`.

To upgrade your app or library to use the new schema, replace the version in
the `$schema` value with `2-0-0`.

- **Breaking change:** It is now an error to use bug numbers (rather than URLs)
in ping definitions.

- Add the line number that metrics and pings were originally defined in the yaml
files.

- BUGFIX: Linter output can now be redirected correctly (1675771).

- **Breaking change:** `glean_parser` will now return an error code when any of
the input files do not exist (unless the `--allow-missing-files` flag is
passed).
- Generated code now includes a comment next to each metric containing the name
of the metric in its original `snake_case` form.
- When metrics don't provide a `unit` parameter, it is not included in the
output (as provided by probe-scraper).