Snippet completion for DirectiveAttributes

[ryanbrandenburg]

Describe the bug: In https://github.com/dotnet/razor-tooling/issues/4436 we added support for Snippet completion of HTML and Component attributes. To complete the feature we need to do the same thing for DirectiveAttribute completion. That gets complicated though since some DirectiveAttributes like @bind don't go directly into @bind="|", but rather @bind:<someOtherString>, which may in turn go into a ="|".

[NTaylorMullen]

If we get enough feedback in this area we will raise the priority of fixing this sooner.