Consider warning about storing VSCode in ~/Downloads

[hardy-ethan]

What
VSCode may think the system is read only if it is stored in ~/Downloads due to quarantining. This behavior is "fixed" by moving VSCode to the Applications folder.

Even though we tell people to install via homebrew, it might be nice to have a warning for people who already have it installed through a different (be it pre-empting the tutorial, or having prior experience).

Where
Maybe a pitfall under the homebrew install command.

Why
An installation with this issue was observed to not be able to compile properly. It also may cause VSCode to not be able to update itself.

References
microsoft/vscode#7426 (comment)
Squirrel/Squirrel.Mac#182 (linked on the above)

[awdeorio]

I've also noticed this problem even in EECS 485. You ALSO end up with VS Code "installed" multiple times, e.g., ~/Downloads/VSCdoe.app and ``~/Downloads/VSCdoe 1.app`.

I like the idea of a pitfall.