[css-anchor-position-1] 各種更新

Use newly-defined force-hidden value for visibility hacking
w3c/csswg-drafts@235852f

editorial: Move sections around for better coherence
w3c/csswg-drafts@235852f

Clarify validation of anchor() and anchor-size()
w3c/csswg-drafts@dcd4fc2

Copy over the 'penultimate CB must precede the abspos' text from the next bullet point to cover similar cases here.
w3c/csswg-drafts@cc3c3ef

Restore accidentally dropped text about CB ordering in one case.
Add missing keywords to <<position-area>> <dl>
w3c/csswg-drafts@c46a6e0

Consistent position of self prefix
w3c/csswg-drafts@e874077

implicit anchor of pseudos is the originating element
w3c/csswg-drafts@f36a6e1

Center track alignment is center.
w3c/csswg-drafts@0527da0

Specify computed value of 'position-area'
w3c/csswg-drafts@af259e6

editorial: Better structure for position-area prose
w3c/csswg-drafts@475c797

Clarify that strongly hidden doesn't affect computed value
w3c/csswg-drafts@1b85aac

Tweak wording in note
w3c/csswg-drafts@a6de115

Author: triple-underscore