[core] Provide a way to customize which elements get created

[hmans]

Right now, once you import "three-elements", all of Three.js will be imported, and custom elements will be generated for all of its (constructible) members. This is fine for quick hosted examples, but makes it impossible to just selectively import parts of Three.js, or perform tree-shaking in a build pipeline that offers this.

Suggestion:

• Move the meat of the current three-elements package to a new package @three-elements/core and make importing that package side-effect free
• Provide a way to selectively generate elements for a list of user-provided Three.js classes
• (Maybe use the opportunity to also allow the user to customize the naming of the tags, eg. use a t- prefix instead if a three- one)
• Reduce the three-elements package in scope so it just imports core and performs the brute-force initialization it's been doing so far.

This way, if you just want "everything", you can keep importing from "three-elements"; if you don't, you'll work with @three-elements/core and explicitly ask it to set up elements for you, following your configuration.

[hmans]

There's some good discussion in this webcomponents-cg issue that suggests we should think about this some more before doing anything crazy, so I'm deferring this to the next milestone.