allow specifying priority in pixi project channel add

[minrk]

Problem description

Since channel order/priority matters, it would be nice if pixi project channel add CHANNEL could specify the priority of the channel being added.

The following would work:

pixi project channel add [--priority N] CHANNEL

to generate the toml:

channels = ["conda-forge", {channel = "CHANNEL", priority = N}]

To compare, conda config --add channels CHANNEL adds the channel as top priority by default, and conda config --append channels CHANNEL adds at lowest priority. pixi project channel add only does the latter, not matching my expectations coming from conda. Conda's default is also more often what I want, since one of the main reasons I use channels other than conda-forge is to test out builds of packages not yet on conda-forge in a custom label. This can't be set up easily without manual edits of the toml file.

[ruben-arts]

Good idea, PR's are welcome!