Build: Support a drop-in project for debugging

Currently, we support moddable-platformer or threadbare. But frequently, we want to load in a custom user project for debugging.

We should support a `custom` project that launches out of an arbitrary directory with a param name.

Start by locating the existing moddable-platformer and threadbare project launch configuration, then trace how a project name and directory are selected for debugging. Add the custom project path and parameter behavior described in the issue, and verify that an arbitrary user directory launches successfully without breaking the existing projects.

Written by the indexing model from the issue text.