Stabilize `edition` key and add `cargo new --edition`

Stabilize `edition` key and add `cargo new --edition`

This commit stabilizes the `edition` key in `Cargo.toml`, both in the
`[package]` section and inside subtargets. Additionally the `cargo new` and
`cargo init` subcommands have been enhanced with a `--edition` flag to allow
explicitly specifying the edition to be generated.

This commit does not yet change the default edition that's generated.

Closes #5980