[meta] Improve the accessibility of Drupal Canvas

In this meta issue the accessibility group has started to gradually add accessibility related issues for Drupal Canvas that were identified and discussed. In addition, already existing issues got added to the list as well. It was tried to keep each child issue tightly scoped.

Remaining tasks

#3563358: Improve the accessibility of text and search fields
#3459098: [META] Define strategy for keyboard shortcuts
#3565786: The hotkey for duplicating a component in the layers section is broken
#3566199: Improve the accessibility of select elements
#3566223: Improve checkbox elements
#3566244: There are redundant sets of items in the aural interface
#3566467: Improve the available landmarks and headings of the Canvas UI
#3566475: The label of the green "Published" pill has a too low color contrast
#3566540: Empty slots have a too low color contrast against dark backgrounds
#3567547: A skip to link functionality is missing