Feature idea: lint workspace dependencies

[yaymukund]

Hi Timon, big fan of your crates, had this idea:

1. In cargo_fmt_manifest.toml:

   workspace-only-dependencies = [
       "clap",
       "crossterm",
       "tokio",
   ]
   

2. This should now be an error:

   [dependencies]
   clap = "4.1"
   

   e.g.:

   Error: clap is defined at the workspace level, yet its version is specified as "4.1". Please specify it as clap = { workspace = true }. See:

   https://doc.rust-lang.org/cargo/reference/workspaces.html#the-dependencies-table

   Hmm, the error message should probably also mention the workspace-only-dependencies array.

What do you think?