Handle shortcuts for actions like copy/paste when embedded inside VS Code

[DanTup]

On macOS, short-cut keys like Cmd+C, Cmd+V don't work in embedded iframes. This happens because VS Code calls setIgnoreMenuShortcuts(true) and disables the OS/browser functionality for these actions. For the top-level webviews, it handles keypresses, passes them up to VS Code, and VS Code then passes back down commands for copy/paste/etc.

However for our nested iframes, this keypress handling (and command passing) does not occur.

I wasn't able to find any way to fix this inside VS Code (it can't reach into our iframes because they're a different origin) but I found that we can simulate what it's doing itself. I thought I'd tried this in the past and determined this wouldn't work, however it seems to work fine today (maybe I was trying to handle the Copy/Paste events rather than the keys? 🤔)

I can't write automated tests for this because we need to verify the actual behaviour of pressing keys, but I've tested on both my Mac and Windows (it already worked on Windows and doesn't need this, but I've left this code to run for all platforms in case there is a difference between using VS Code's native menus vs not) by using dart.customDevTools and in both of them, I can copy/paste/select-all/undo.

Fixes Dart-Code/Dart-Code#3488
Fixes #7253
Fixes #9435
See #8190 (I don't know if we still want to do other work here)
See microsoft/vscode#129178 (comment)

cc @elliette

[elliette]

One question but LGTM!

I'm going to file a bug for the flaky integration tests (re-running them now).