Iframe based webview prototype

* add window.disableMenuBarAltBehavior option

* enable alt behavior when menuBarVisibility is set to 'toggle'

* rename 'window.disableMenuBarAltBehavior' to 'window.disableCustomMenuBarAltBehavior' and change its description

* don't affect mnemonics

Diff https://github.com/xtermjs/xterm.js/compare/566f594...cdc9f79

- Addon webpacking
- Docs
- Fix DOM cursor blink
- Remove some IE workarounds

Fixes #75534

Terminal race between process exit and on ready listeners

Mark events as readonly in VS Code api

Fixes #75509

Fix LANG env var for Czech

Part of #75091

Implement API to get default shell

fixes #72531

related to #72531

Don't use heap service for signature help