Fix <form> elements in invalid AMP contexts

[ericlindley-g]

Some developers want to use AMP components and v0.js when the page itself isn't valid AMP. This is generally unsupported, and causes a couple problems, including:

The runtime breaks some non-AMPHTML elements. An example of this is <form> — AMP requires that the amp-form script be included in the page and for forms to be implemented per the amp-form spec for them to work.

This issue is to fix (2), so it's possible to use <form> elements inside of the context of AMP but not necessarily with <amp-form>, and not necessarily in the way that <amp-form> expects you to. This would necessarily invalidate the page from an AMP validator perspective, so it would not be served by an AMP cache.

/to @alabiaga who I believe was thinking about working on this, and
/cc @choumx @dvoytenko @jridgewell @westonruter @amedina for visibility

Note: there's already an open feature request for related invalid AMP issues.