Add a `cabal vendor` command

**Describe the feature request**
Add a `cabal vendor` command that vendors dependencies automatically.

**Additional context**
From a question on how to do automatic vendoring with cabal, #10934.

Start by reading this feature request together with the related question in #10934 to understand the automatic vendoring use case. Identify the cabal-install command entry point and dependency-vendoring behavior involved; done means a `cabal vendor` command can automatically vendor dependencies as requested.

Written by the indexing model from the issue text.