Fix S6931 FN: Route templates starting with ~/

[antonioaversa]

Route templates starting with ~/ seem to have an equivalent behavior to the ones starting with /: see here:

In the preceding code, the Index method templates must prepend / or ~/ to the route templates. Route templates applied to an action that begin with / or ~/ don't get combined with route templates applied to the controller.

If so, the following is a False Negative:

public class A2Controller : Controller
{
    [Route("~/B")]
    public IActionResult B3([FromQuery] Person person) => Content("Hello");
}

since the following is a True Positive:

public class A2Controller : Controller // Noncompliant
{
    [Route("/B")]
    public IActionResult B3([FromQuery] Person person) => Content("Hello");
}

Alternative: We may also decide to consider ~/ as a more explicit way than /, for the user to say that the route should actually be attached to the web-application route, and not to the controller one. In that case, we should add ~/ as an exception for S6931.

Test case: https://sourcegraph.com/github.com/bitwarden/server@b9049cd699e8a80c585346f9c53caa40f38133f2/-/blob/src/Api/Controllers/InfoController.cs