Add CLI command or option to be able to list all platforms that pixi supports

[matthewfeickert]

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Problem description

Bumping https://github.com/prefix-dev/pixi/discussions/1742 to a feature request, pixi knows what valid platforms it supports

https://github.com/prefix-dev/pixi/blob/4e12e86a2c4a5c74791fd8f2abd9d85b85c4895f/schema/schema.json#L676-L732

however for a user this information would be very useful to be able to access from the command line. It would be great if some CLI command or existing command option could be added to show all valid platforms that pixi knows about.

Use case

From habit and looking up the pixi docs I know that the typical platform list that I want to use in my projects is generally

platforms = ["linux-64", "osx-64", "osx-arm64", "win-64"]

However, if I wanted to think about extending support to other platforms I currently don't know how to find these (I don't think as of v0.27.0 they are even listed in the docs). At the moment the only way that I know how to get a list of all valid platforms without looking in the source code is using this clever hack (https://github.com/prefix-dev/pixi/discussions/1742#discussioncomment-10274825) of intentionally trying to add an invalid platform to the project as pixi will then list out all the valid platforms. It would be great if a user could request this information explicilty.