cargo add --workspace for workspace deps

[tqwewe]

cargo add currently supports the following "sections":

Section:
      --dev
          Add as development dependency
          
          Dev-dependencies are not used when compiling a package for building, but are used for compiling tests, examples, and benchmarks.
          
          These dependencies are not propagated to other packages which depend on this package.

      --build
          Add as build dependency
          
          Build-dependencies are the only dependencies available for use by build scripts (`build.rs` files).

      --target <TARGET>
          Add as dependency to the given target platform

It would be nice if an additional section was added called --workspace, which adds the dependency under [workspace.dependencies].

[epage]

FYI cargo add now lives in cargo

The relevant issue for this is rust-lang/cargo#10608