Add support for @namespace directive for components

[ajaybhargavb]

I'm not sure if this was intentional but looks like we don't really support namespace directive in component documents. To support this, we'll need to add a ComponentNamespaceDirective similar to ComponentPageDirective.

Although the behavior of this directive isn't any different from regular Mvc/Pages, we can't use the same type because the existing NamespaceDirective resides in Mvc.Razor.Extensions and we can't move it over to Razor.Language because it is not supported in 1.x apps.

Note: This is a prerequisite for supporting namespace directives with usings in #5577

cc @rynowak

[rynowak]

Could we move it over to Razor.Language but only add it for Razor >= 2.0?

Either way we should support this.