Adopt Edit Context

[hediet]

Follow up of #204371 (exploration done in #207699).

This issue is about:

• When available, use EditContext to react on user input
• Use existing text area or new div elements to render content for screen-reader users

Advantages:

• Very likely fixes many IME bugs
• Simplifies IME handling in code

Disadvantages:

• EditContext is not available on all platforms, in particular not on electron by default
• EditContext does not improve accessibility directly (indirectly, by allowing to use divs to render the screen reader content)
  • text area code is complicated and needs to be adjusted for screenreader support

[alexdima]

@aiday-mar Breaking things down (from our planning discussion):

• Accessibility
  • Get NVDA set up and working well (Caps-Lock and Insert don't work out-of-the-box with Parallels)
  • Learn about how to operate NVDA
    • browse vs focus mode
    • system caret vs review cursor
    • jump to previous / next paragraph without moving the system caret
  • Understand the current pagination based text strategy
  • Propose a new implementation
  • Implement using <div> text content and browser selection
• IME
  • Try out IME scenarios from https://github.com/microsoft/vscode/wiki/IME-Test
  • Fix up the wiki page if it is out of date or incomplete
• Try to see what others such as Outlook Web or Word Online do