quarto install extension from branch

[jmbuhr]

It would be handy for extension development and testing if you could use some syntax to specify a branch from which to download an extension via e.g. quarto install extension <username>/<reponame>@<branch-or-tag>

[jjallaire]

The tag version of this works now, the branch version should work soon (see #1836)

[mcanouil]

Closing as completed.