Make `cargo lambda new` work in an existent project

[calavera]

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That subcommand is great to quickly scaffold new functions, however, it only works for new projects.

It'd be very nice if it also worked on existent projects, where it'd add the function code to an existent package.

[messense]

Cargo use a separate cargo init command for this: https://doc.rust-lang.org/cargo/commands/cargo-init.html , I think cargo-lambda should follow this common pattern.

[calavera]

I think cargo-lambda should follow this common pattern.

I'm all for this. I forgot that cargo-init was a thing.