Expose and make global the node_modules folder

[anoblet]

The Home Assistant frontend relies heavily on custom elements. One of the shortcomings of this is that once defined, custom elements cannot be upgraded or replaced.

An example scenario:

1. Home Assistant imports @material/mwc-button@^0.20.0.
2. A developer creates a custom card which imports @material/mwc-select@^0.25.3

Both elements use @material/mwc-ripple. When the page loads, @material/mwc-ripple is not deduped and tries to define itself twice (@material/mwc-ripple@^0.20.0, and @material/mwc-ripple@^0.25.3). This will crash with the error:

Failed to execute 'define' on 'CustomElementRegistry': this name has already been used with this registry

If we were to expose the node_modules folder (the same way addon, backups, config...) are exposed, we could dedupe these dependencies allowing for developers to upgrade/install new elements that may or may not share a common dependency.

This is a known issue in the custom element community, and there are current initiatives on figuring out how to eliminate these conflicts including scoped custom element definitions and using IFrames to isolate the definitions though those may be far off.

Here's a discussion recently started in the lit repository, describing the issue and possible solutions: lit/lit#2432