XmlCompletionItemManager doesn't enable filters

[KirillOsenkov]

When no text is typed in completion, all filters are disabled, because it doesn't create CompletionFilterWithState with IsAvailable = true for available filters.

[KirillOsenkov]

The current version is pretty close to the original here:
https://github.com/microsoft/vs-editor-api/blob/cc54ccf435221210660b51f0f9ca49c0c99407fa/src/Editor/Language/Impl/Language/AsyncCompletion/DefaultCompletionItemManager.cs

I'm guessing it adds extensibility points so that MSBuildCompletionItemManager can inherit from it and provide async updates for NuGet search

[KirillOsenkov]

This quick and dirty fix seems to fix it:

(makes all filters available by default if no text was typed to narrow down the list)