[WIP] Move eligible `@keyframes` rules to `styel[amp-keyframes]`

[kienstra]

Summary

• When a @keyframes rule has valid properties for the style[amp-custom], this moves it there.

• Still, this need to be reworked to consider @media queries. If a @keyframes is wrapped in a @media query, it should also be wrapped in it when it's moved to the style[amp-keyframes].

Fixes #1626

Checklist

• [x] My pull request is addressing an open issue (please create one otherwise).
• [x] My code is tested and passes existing tests.
• [x] My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

[kienstra]

Sorry, forgot to open this as a draft, instead of using [WIP].

[kienstra]

This is on hold, per https://github.com/ampproject/amp-wp/issues/1626#issuecomment-589365559

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