Support for dynamic OnAutoInsert capability registration

[mgoertz-msft]

Is your feature request related to a problem? Please describe.

OnAutoInsert is not part of the public LSP spec. However the client for the RoslynLanguageServer does send textDocument/_vs_onAutoInsert requests. Currently, this feature only pays attention to the statically registered server capabilities/options and only the C# language server handles these requests. XAML registers its capabilities dynamically with different request options and therefore cannot handle OnAutoInsert.

Describe the solution you would like

OnAutoInsert needs to become a full DynamicFeature to allow static and dynamic DocumentSelector based capability registration.

Applicable Scenarios

XAML editing in VS Code.

Describe alternatives you've considered

n/a

Additional context

n/a

[mgoertz-msft]

FYI, I'm already working on this. :)