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
````

I don't think this check is necessary since it calls `_beginSession()`.