Add new entrypoints to webcomponentsjs for the 'platform' polyfills.

[bicknellr]

Fixes #386.

I think the names for these bundles need some bikeshedding. Having these file names in the package is probably going to be very confusing for consumers:

bundles/webcomponents-js.js
bundles/webcomponents-dom.js
src/entrypoints/webcomponents-js-index.js
src/entrypoints/webcomponents-dom-index.js

pf is used in the other bundle names for 'platform' but now that they're split into two, we need some new separate name for each. js and dom were just the most obvious, but clearly aren't helpful if you're just looking around at the files in this package.

[aomarks]

Looks fine, I don't have an opinion about the names.

Can you remind me why we need to make these entrypoints for lit next (or update PR description)? I don't remember the discussion.

[bicknellr]

Shady DOM and the custom elements polyfill both use the @webcomponents/webcomponents-platform package in some of their tests but we no longer have @webcomponents/webcomponents-platform as a distinct package in our source after merging all of the polyfill repos, so we can't update them (without recreating the package). These entrypoints will give us a place to put the newer polyfills that we're adding for Lit (append, remove, etc.) so that we can use them without importing the entire webcomponentsjs bundle in the tests.

[kevinpschaaf]

bundles/webcomponents-es6.js (to disambiguate from webcomponents.js) and
bundles/webcomponents-dom.js

?

[kevinpschaaf]

Or
bundles/webcomponents-es6-shims.js
bundles/webcomponents-dom-shims.js

[bicknellr]

I think I might just go with the pf_ prefixing thing mentioned in chat since IDK if these are going to be strictly ES6.

[bicknellr]

Ok, updated, anyone opposed?