When the hostname can not be inferred, suggest the --hostname option

[Fil]

trafficstars

ref. https://github.com/observablehq/framework/discussions/1535

I'm not sure how we arrive at this point, but we should definitely not print out http://undefined:3000 and instead print out a message suggesting to use an explicit --host.

[mbostock]

I can’t see how this is possible given that the --host argument has a default of "127.0.0.1".

[mbostock]

(Also, the argument is --host; hostname is the name of the internal option but that’s not part of the public API.)