improve accessibility of inline completions

[meganrogge]

This:

• Adds an inline completion accessible view
• Adds a hint to open that (EditorOption / verbosity setting controlled)
• Changes the accessible view's aria label from a hint to open the accessibility help menu to one that suggests using shift+tab to explore the available actions, citing disabling this hint as an example.
• Enables navigating to the next/previous completion from within the accessible view
• Allows accepting the completion from within the accessible view
• Announces not just the partial text, but the full completion to the screen reader - this makes more sense IMO

fix #186307