[SAY-5]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and discussions and couldn’t find anything or linked relevant results below
• I made sure the docs are up to date
• I included tests (or that’s not needed)

Description of changes

Closes #15.

parse5 only sets content on template nodes in the HTML namespace, so <svg><template> or <math><template> hits one(state, undefined) and throws. hastscript also unconditionally moves a template's children into .content regardless of namespace, so the fix has to put those children back on the element when there is no real parse5 content to use instead.

Added a test for both the svg and mathml case.