Find a better way to edit templates

[molenzwiebel]

Relevant comments in template-content.ts: https://github.com/ace-lol/Ace/blob/master/src/hook-providers/template-content.ts#L10-L13.

Currently, we wrap HTMLTemplateElement.content and give plugins the opportunity to do their initialization/changes there. However, the scope of the hook is fairly big, even if plugins behave and unregister their hook after applying their changes. Ideally, I would like to edit the HTML after insertion, but that gives a whole other range of problems/things to consider.