Feature layers should be fully independent

[kytans]

Currently features are all put in a single image that is then added in a layer, which separate layers per feature only for running install.sh.

This means that changes to any feature or adding a feature cause a full rebuild of the devcontainer, and also means that creating a variant of a devcontainer with an extra feature will rebuild all the features.

Instead, each feature should be added in a separate layer, which would mean that only features that are listed later than the changed features need to be reran while earlier features are cached.

This would speed up some use cases considerably and is much cleaner.