`@std/net`: Add `isPortAvailable`, etc.

[yuhr]

Is your feature request related to a problem? Please describe.

Currently, features of @std/net is very limited; only 1 API i.e. getAvailablePort.

Describe the solution you'd like

Provide some useful stuff as seen in https://deno.land/x/port@1.0.0.

Describe alternatives you've considered

Using https://deno.land/x/port@1.0.0/mod.ts

[Xavrir]

Happy to take this one. To start small, I'd add a single function alongside getAvailablePort:

function isPortAvailable(port: number, options?: { hostname?: string }): boolean

It returns true if the TCP port can be listened on, false on Deno.errors.AddrInUse, and rethrows anything else. hostname defaults to 0.0.0.0. I put it in unstable_is_port_available.ts (the @std/net/unstable-is-port-available export), and the JSDoc is upfront that the result is racy, so it points people at port: 0 / getAvailablePort for cases where they own the listener.

It's already written, with tests for the available, in-use, hostname, and non-AddrInUse error cases, and it passes deno task lint, deno fmt --check, and deno test. Can open the PR whenever.

Does that shape look right to you? I can fold in getAvailablePorts(count, options?) too if that's useful, or keep this one small.