Improve typing for ApplicationV2 `_renderHTML` and `_replaceHTML`

[JPMeehan]

1. _renderHTML and _replaceHTML are blank, protected functions meant to be defined by the rendering mixin, e.g. HandlebarsApplicationMixin
2. The return of _renderHTML feeds directly into _replaceHTML as its first parameter
3. Initial attempt was result: Awaited<ReturnType<this["_renderHTML"]>>, but Private or protected member '_renderHTML' cannot be accessed on a type parameter.