[css-animations-2] 各種更新

Nope, the trigger state actually does live on the trigger, duh.
w3c/csswg-drafts@3232f5c

editorial]:fix linking errors
w3c/csswg-drafts@7daaa2f

Separate trigger concept from trigger instance. Define that event triggers can be defined to have enter/exit pairs, like timelines.
w3c/csswg-drafts@4ec10ae

initial value of the exit ranges is 'auto', to match the start ranges (copypaste error)
w3c/csswg-drafts@680e999

Author: triple-underscore