serving/helloworld-nodejs: Respect the PORT env var

[grayside]

Works towards #456 and makes a minor README update follow-up to #443

Proposed Changes

• Respect the $PORT environment variable as a default behavior
• Replace use of var with const
• Add exclamation mark back to the message!
• Update the duplicated sample code in the README to match changes
• Note that the default behavior if no $TARGET variable is set will be 'Hello World'

[samodell]

/approve
/lgtm

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: grayside, samodell, steren

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [samodell]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment