Support invoking sea-orm-cli as a Cargo subcommand

[onenewcode]

Motivation

Closes #3151.

It's currently not possible to run the CLI as a standard Cargo subcommand (cargo sea ...), which prevents users from defining Cargo aliases for long commands, e.g.:

[alias]
m-entity = "sea generate entity --database-url ..."

Changes

• Add a cargo-sea binary target (reusing the existing src/bin/main.rs), so Cargo can discover it as the sea subcommand.
• When invoked as a Cargo subcommand, Cargo injects the subcommand name (sea) as argv[1]; strip it before parsing, but only under a cargo- binary name, so standalone sea / sea-orm-cli invocations are unaffected.

Usage

cargo install sea-orm-cli        # also installs the `cargo-sea` binary
cargo sea generate entity -u sqlite://bakery.db -o out
cargo sea migrate up