Added support for localizable routes

[tomasherceg]

This PR adds support to define multiple route URLs for the same route that will apply in different request cultures.
A default route URL, which is applied for any unmatched language, is required.
The primary motivation for this is using the same route name for all language versions of the same page. Without that, the route name for each language would have to be different and thus dynamically computed for all RouteLink controls in the application).

config.RouteTable.Add("FeatureSamples_Localization_LocalizableRoute", "FeatureSamples/Localization/LocalizableRoute/{lang?}", "Views/FeatureSamples/Localization/LocalizableRoute.dothtml",
    localizedUrls: new LocalizedRouteUrl[] {
            new("cs-CZ", "cs/FeatureSamples/Localization/lokalizovana-routa"),
            new("de", "de/FeatureSamples/Localization/lokalisierte-route"),
    });

The RouteLink control was extended with the Culture property to generate a link to the specified route for a different culture (to allow easy language switching).

The route matching assumes that the request culture is already set.
Detection of the language must be done at the beginning of the request pipeline:

• In ASP.NET Core, the easiest way is to use the standard Request Localization middleware.
• In OWIN, there is no built-in mechanism. It is sufficient to write a middleware, which sets CultureInfo.CurrentCulture and CultureInfo.CurrentUICulture, but you must also call context.Set(HostingConstants.OwinDoNotSetRequestCulture, true); to prevent DotVVM from overwriting the current culture to configuration.DefaultCulture.

LocalizablePresenter is not supported for this feature as it detects the request culture too late.

Also, the localization is currently unavailable for URL segments defined in route groups.

[exyi]

Suggested change

	public bool TryGetValue(string routeName, out RouteBase? route)
	public bool TryGetValue(string routeName, [MaybeNullWhen(false)] out RouteBase? route)