Use a `RTICScope.toml` instead of `Cargo.toml` metadata

[tmplt]

A lot of required fields are now expected in cargo manifest metadata. I predict more will end up there. It may be a better idea to read these parameters from a RTICScope.toml file instead.

In either case we need to add a cargo rtic-scope init command that adds a skeleton file (or fields in Cargo.toml metadata).

An RTICScope.toml file will also draw more attention than a Cargo.toml when inspecting a crate.