Change pixi add to work comfortably with packages and workspaces.

[tdejager]

@nichmor and I worked on a little design to solve pixi add for packages vs workspaces.

We want the user to have a comfortable CLI to add packages to both workspaces and packages.

A package can be added to [dependencies], [package.run-dependencies], [package.host-dependencies], [package.build-dependencies] and all it's target and feature(only on workspace) variants.

• pixi add package-x:
  Has a little automation in it to simplify the UX for the user.
  • If there is a [workspace] then add the package-x to [dependencies]
  • If there is ONLY a [package] then add it to [package.run-dependencies], we do this for easier onboarding and UX comfort. (throwing an error here would result in a user needing to understand run vs host vs build before they can get something working).
• pixi add package-x --host/--run/--build will add it to [package.{type}-dependencies] IF it already has a [package] table. Otherwise, throw an error.
• pixi add package-x --workspace will skip the automation and throw an error if there is no [workspace]. Conflicts with --host --run --build.
• pixi add --host --target unix will add it to [package.target.unix.host-dependencies]

Originally posted by @ruben-arts in #2807

I've extacted this into a seperate issue, because the other will close, once the PR that fixes it is complete.