fix(console): declare tokio-console bin as default-run (#379)

Currently, the documented `cargo-run` in README no longer works, as now
there are two binaries:

```console
$ cargo run
error: `cargo run` could not determine which binary to run. Use the `--bin` option t
o specify a binary, or the `default-run` manifest key.
available binaries: tokio-console, xtask
```

This branch declares the `tokio-console` binary as the [`default-run`]
value in the `Cargo.toml`, so it is now run by default by `cargo run`.

Alternatively, we can just update the README line with `cargo run --bin
tokio-console`

[`default-run`]:
    (https://doc.rust-lang.org/cargo/reference/manifest.html#the-default-run-field)

tokio-console/Cargo.toml

@@ -7,6 +7,7 @@ edition = "2021"
 rust-version = "1.58.0"
 authors = ["Eliza Weisman <eliza@buoyant.io>", "Tokio Contributors <team@tokio.rs>",]
 readme = "README.md"
+default-run = "tokio-console"
 homepage = "https://github.com/tokio-rs/console/tree/main/tokio-console" 
 description = """
 The Tokio console: a debugger for async Rust.