feat: extends manifest to allow for preview features

[tdejager]

Description

This PR adds the ability to allow the use of preview features, we want to use this to enable certain features that should be enabled on the manifest level. The first one being, the workspace and build functionality. The idea behind these features is that we can still break and test out new workflows while being in preview, but we do like to get some hands-on testing for these features.

Say for example we allow for a new solver (don't worry we are not really doing this :)), you could then specify:

[project]
# ... more metadata
preview = ["new_solver"]

Alternatively, to allow for all preview features one can use preview = true.

Notes

I've modified the schema and the documentation for this change. No features have been specified as of yet, but it should be relatively straightforward by adding an enum value to the KnownPreviewFeatures enum.

Testing

I think most of the testing for this is covered in CI for now. Best to look at the API in project and see if this is something that looks good for now.

[nichmor]

I like it

[ruben-arts]

Looks good, a bit hard to test as there is no preview feature yet. But works as expected so far.

I do think it's good to use serde_ignored for the warning to streamline that workflow over pixi's parsing.

[tdejager]

Also checked out if I could do with just an #[serde(untagged)] in the enum list, however, this messes up the error messages again, and flattening KnownPreviewFeature and PreviewFeature had a bit of bad DX when adding and removing from the enum which we would probably do quite a bit.

Now you can just add/remove to the enum and everything just works. So would rather keep like this.