Explainer for Shadow Parts support for native shadow DOM

[aomarks]

Document how we will support Shadow Parts in browsers that do support native shadow DOM, but don't support ::part (e.g. Safari).

[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!