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
````

This is where we're initializing the default value regardless of whether the prefs are available or not. This is why I use it on the 3rd param to `definePreference` on line 68. But I can see that you want the other way around. I'm fine to hard-code `true` for the 3rd param to `definePreference`. Then if we ever want to flip the default, we have to update these two places at the same time.