new: use top-level [workspace] Cargo.toml

[nick1udwig]

Problem

#90: use top-level Cargo.toml in templates with [workspace] field.

Solution

Resolves #90 by adding that Cargo.toml.

Docs Update

N/A

Notes

Please try creating a template and let me know if this change makes the Rust analyzer happier. E.g., something like

# Install this branch's kit
cargo install --path .

# Create the package from template.
kit n foo -t file_transfer

# Edit files like file_transfer/file_transfer/src/lib.rs or file_transfer/worker/src/lib.rs

[jaxs-ribs]

Tested, and LGTM!