Manually handle focus interaction between Flutter and CodeMirror

[parlough]

I'm not sure if this is the best or correct solution, but I tried out the following to prevent focus from being shared and enabling tab to work in the CodeMirror editor. I added Esc and Shift+Esc to escape the editor, but perhaps there's something more standard?

Let me know what you think of both the implementation and the behavior. Thanks!

Potentially fixes #2852

Staged: https://parlough-dart-dev.web.app/

[devoncarew]

I'm wondering whether we actually need the escape / shift-escape keybindings...

[devoncarew]

Do we need this? I wonder what the expectations are wrt focus traversal and text editors. I think that if the focus ever escapes the text editor, people would expect the normal focus traversal until the focus was again captured by the text editor. But do users expect to have a keybinding that lets them move the focus out of the text editor?

[parlough]

I think we need some key binding like this as otherwise users have no easy way for focus traversal to escape CodeMirror with just the keyboard.

[parlough]

I think I'll remove these for now, as that still leaves us in a better spot than currently, then investigate adding something like the Esc+Tab that CodeMirror 6 uses as it is likely what developers are becoming familiar with.

[devoncarew]

I think we should also make the editor area the default focus. In a Focus subclass, I believe this is the autofocus param (I have not looked to see exactly how that would apply to this PR). Or potentially, an explicit FocusScope.of(context).requestFocus() somewhere during startup.

[dgreensp]

CodeMirror listens for "escape followed by tab" as a way to tab out of the code editor if the tab key has been bound to a command: https://codemirror.net/examples/tab/