Bugzilla

I opened a PR on the docs page, so that users know what to expect: https://github.com/mozilla/remote-settings/pull/811

In the short term:

- `env.version`
- `env.channel`
- `env.locale`
- `env.appinfo.OS` (_missing_)
- `versionCompare()` 

And then there will be a mid/long term work of normalization across gecko and A-S. But first, **please let stop the bleeding**, please do not add new custom fields that would be A-S only. 

Adding a new fields should be prettier straightforward, and there should be very few reasons to go platform-specific:
1. Put Remote Settings stakeholders in the loop (would allow to avoid disparities like casing, etc.)
2. Add the field on Gecko [in this file](https://searchfox.org/mozilla-central/rev/dd8b5213e4e7760b5fe5743fbc313398b85f8a14/toolkit/components/utils/ClientEnvironment.sys.mjs#31) (_even as constant or empty value_)
3. Mention the field in the [official JEXL documentation](https://remote-settings.readthedocs.io/en/latest/target-filters.html)

I opened a PR on the docs page, so that users know what to expect: https://github.com/mozilla/remote-settings/pull/811

In the short term:

- `env.version`
- `env.channel`
- `env.locale`
- `env.appinfo.OS` (_missing_)
- `versionCompare()` 

And then there will be a mid/long term work of normalization across gecko and A-S. But first, **please let stop the bleeding**, please do not add new custom fields that would be A-S only. 

Adding a new fields should be prettier straightforward, and there should be very few reasons to go platform-specific:
1. Put Remote Settings stakeholders in the loop (would allow to avoid disparities like casing, etc.)
2. Add the field on Gecko [in this file](https://searchfox.org/mozilla-central/rev/dd8b5213e4e7760b5fe5743fbc313398b85f8a14/toolkit/components/utils/ClientEnvironment.sys.mjs#31) (_even as constant or empty value_)
3. Mention the field in the [official JEXL documentation](https://github.com/mozilla/remote-settings/blob/ee84d042261c27cbe7c8c433f646183d82dde3a9/docs/target-filters.rst)