Clarify the scope of embedded extensions

Bug description

From https://quarto.org/docs/extensions/formats.html#extension-embedding.

Steps to reproduce

quarto create extension format:html myhtml --no-prompt --no-open
cd myhtml
quarto add mcanouil/quarto-iconify --embed myhtml --no-prompt
echo "{{< iconify simple-icons:quarto >}}" >> template.qmd
quarto render template.qmd --to html

Expected behavior

In the example, Iconify shortcode should be recognised in the template when rendering as it's the case in Quarto 1.3.

Actual behavior

Embedded extensions are no loner available to documents.

Your environment

• macOS

Quarto check output

Quarto 1.3 is the latest known version to work.