Add extension setting to toggle undefined variable warning for global variables

[okesokes]

Added an extension setting to toggle undefined global variable warnings on and off, as sometimes the 'Possibly undefined variable' warning for global variables with its underlining clutters the code and VS Code 'Problems' list.

The default setting is 'false', i.e. "don't show undefined global variable warnings" as this is most likely the preferred one for most developers.

[SkaceKamen]

I personally think this should be ignoreGlobalUndefinedVars and should default to false. What's your use case, why do you have an issue with undefined global variables?

In my projects this is usually fine.