Enable authors to specify SXG-only tags about sxg-rs HOT 4 OPEN

and/or we could have a simpler API for the common use case. Something like:

<meta name=declare-sxg-var>

becomes something like:

<script>window.isSXG = true</script>

from sxg-rs.

On further thought, we should definitely at least do the first one (probably both). And it should delete the template tag on non-SXG.

This covers another use case: being able to include JS & templates (they can be nested) for lazy-loading personalized elements, without affecting the byte size of the non-SXG version (which can personalize via dynamic HTML instead).

from sxg-rs.

Remaining task is to implement on fastly_compute.

from sxg-rs.

Implementing for fastly_compute should be easy now that process_html.rs exists.

from sxg-rs.