Make ShadyCSS Shadow Parts an optional shim

[aomarks]

See https://github.com/webcomponents/polyfills/pull/329/files#r430699466

The most important reason to do this could be users who don't want to pay the byte cost of additional shadow parts support.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically closed after being marked stale. If you're still facing this problem with the above solution, please comment and we'll reopen!