Add information about stylesheets included and excluded in style[amp-custom]

[westonruter]

The following will be included, for example, to list out the stylesheets that are concatenated into style[amp-custom] and also to indicate which stylesheets were excluded for being too large.

<!--
The style[amp-custom] element is populated with:
- link#ampconf-css[rel=stylesheet][id=ampconf-css][href=https://src.wordpress-develop.test/wp-content/themes/ampconf/assets/dist/css/main.css?ver=4.9.6-alpha-42959-src][type=text/css][media=all] (15053 bytes)
- link#jetpack-widget-social-icons-styles-css[rel=stylesheet][id=jetpack-widget-social-icons-styles-css][href=https://src.wordpress-develop.test/wp-content/plugins/jetpack/modules/widgets/social-icons/social-icons.css?ver=20170506][type=text/css][media=all] (207 bytes)
Total size: 15260 bytes

The following stylesheets are too large to be include in style[amp-custom]:
- link#amp-default-css[rel=stylesheet][id=amp-default-css][href=https://src.wordpress-develop.test/wp-content/plugins/amp/assets/css/amp-default.css?ver=1.0-alpha][type=text/css][media=all] (77107 bytes)
-->

This will be very helpful for debugging and it will improve transparency.

[kienstra]

Approved

Hi @westonruter,
Thanks, it's really helpful to see the sources of the CSS in the <style amp-custom>. This should also help with WordPress.org support topics.

The comments appeared as expected when using theme support (Paired Mode and Native AMP).