Created a text anchor system for super_text_layout, and a content anchor system for super_editor. (Resolves #424)#1216

matthew-carroll wants to merge 3 commits intomainFlutter-Bounty-Hunters/super_editor:mainfrom

424_create-content-anchor-systemFlutter-Bounty-Hunters/super_editor:424_create-content-anchor-systemCopy head branch name to clipboard