Setup and host a development / staging set of extensions

[jonkeane]

We should setup a second feed that includes extensions that we are testing out before a formal release. This (combined with the internal feature of having multiple gallery feeds) will let us test out extensions before fully releasing them to everyone.

[dotNomad]

Thinking through the implementation here for staging extensions in this repo:

• We use semver to determine releases, and could mark release with pre-release tags as specified in the semver versioning system: https://semver.org/#spec-item-9
• node-semver (what we use for comparison) has CLI support for including or not included pre-release version in range matching which could tell us which extension list to send a release to (if it has a pre-release tag or not)

There is a question if we want the two lists (staging and non-staging) to work together or not. It one case we could have the staging list contain all releases (including pre-releases) and duplicate the data. On the other side we could have the lists work together and the staging list only contain pre-releases, removing them from the list when officially released so there is no duplication in the Gallery if both are enabled.