feat: infer runtime id from webpack uniqueName option

[DylanPiercey]

Description

Now automatically infers the Marko runtime id (used to allow multiple instances of Marko on the same page) based on the uniqueName option in webpack.

Motivation and Context

This makes it so that apps embedding multiple versions of Marko don't have to configure runtimeId by default.

Checklist:

• [x] I have updated/added documentation affected by my changes.
• [x] I have added tests to cover my changes.