Make `EditContext` the default editor input approach

[alexdima]

We should work towards making EditContext the default.

• Tackle all known issues caused by EditContext
• Do a mini test pass on: mac, linux, windows, web
• Enable it by default in Insiders

[hediet]

Also:

• Get feedback from screen reader users

[aiday-mar]

Concerning the accessibility, I created the following issue #229051 asking Prof. JooYoung Seo and Roberto Perez if they could test the edit context setting with screen readers.