Simple code hints preferences to enable/disable individual/all code hints providers.

This fixes #8173. You can use one or more of the following preferences to turn on/off individual hints provider.
By default, `showCodeHints` is true and you don't need any of the following preferences to use the existing code hints providers. When you want to disable any of them, just add `"codehint.<HINT_PROVIDER_CONSTRUCTOR_NAME>": false` in the brackets.json file.

````
    "codehint.TagHints": false,
    "codehint.AttrHints": true,
    "codehint.CssPropHints": true,
    "codehint.UrlCodeHints": true,
    "codehint.SpecialCharHints": false,
    "codehint.JSHints": true,
    "showCodeHints": true
````

>  if pref is not set (i.e. undefined), then default is true (not false).

Both of these prefs have default values, so this statement is not correct, but there is no UI so user needs to manually set values, so it's still a good idea to compare that returned value is `!== false`.