Recognize altKey correctly on macOS

[AriaMinaei]

• VSCode Version: 1.11.0-insider (6a00291)
• OS Version: macOS Sierra 10.12.3

Steps to Reproduce:

1. Define this keybinding and try it: {"key": "alt+u", "command": "cursorHome"}

What's expected: For cursorHome command to be invoked.
What actually happens: This character gets written: ¨

Atom used to have this problem too, which was solved by this PR: atom/atom-keymap#163

More context: atom/atom#13043

[alexdima]

I have validated that we now (i.e. after PR #22894 aka #17521 -- in today's Insiders build) appear to behave correctly for this key combination. I have tested under the US layout.