Include HTML comments at the start and end of the podlet content

[wkillerud]

To make it simpler when debugging layouts we would like to have some sort of visual cue in the markup where a podlet starts and ends.

A layout can add this manually, but it's a bit tedious. If we include it as a feature in podlets then layout authors don't need to do anything. If we make the comment somewhat structured we can also add features to the browser plugin to help visualise information about the different podlets on a page (podium-lib/dev-tool#79).

[wkillerud]

Suggestion loosely based on the Lit comments.

<!--podlet name=<name> version=<version>-->

<!--/podlet-->