Self-hosted cargo-messages crate

[dherman]

Initial implementation of a self-hosted cargo-messages crate. I'm still in bootstrapping mode so there are key missing pieces:

• test suite
• code review
• full release automation for the @neon-rs namespace, i.e. this whole repo

But this is the first step. The key self-hosting trick is that @neon-rs/cli is bundled with ncc, which will boil away any explicit dependency on cargo-messages. That way @neon-rs/cli-source can have a dependency on cargo-messages, and cargo-messages can have a dev dependency on any published version of @neon-rs/cli, without creating a cycle:

Next steps:

• Configure the npm auth for the repo
• Setup GH Actions automation for publishing cargo-messages to npm, with prebuilds
• Then we'll be able to replace cargo.ts with a dependency on cargo-messages
• Setup GH Actions automation for publishing @neon-rs/cli