[animation-triggers-1] Add animation triggers 1

[ydaniv]

WIP on moving all spec text from css-animations-2 and web-animations-2 into a new dedicated module animation-triggers-1.

• Create initial text layout and write the generic sections.
• Move all text from css-animations-2 with fixes and minor/editorial changes/updates
• Move all text from web-animations-2 and rewrite to fit the new model

/cc @flackr @DavMila @tabatkins @fantasai

[tabatkins]

I think this looks good to go ahead and merge now? No reason to have it sitting in a PR while you're waiting to transfer the WebAnim stuff.

[ydaniv]

Thanks, @tabatkins, for the quick response. I guess that's an option, to merge this and continue on subsequent PR. If no objections I'll merge tomorrow morning (IL time).