* Add optional [warnings | annotations] keyword to nullable directive

* pragma warning <setting> nullable -> nullable <setting> warnings

* Implement /nowarn:nullable

Add initial support for 'notnull' generic type constraint.

See https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-05-15.md
Also fixes #36005.

* intellisense should suggest event after writing readonly in a struct member declaration.

* Avoid repetition of keywords in EventKeywordRecommender

* Enable completion for modifiers allowed for interface members.

* Enable completion of keywords within an interface

* Fixing WhenKeywordRecommender to understand var pattern
* Using block body for test methods
* Adding a comment

Fix wrong logic when is not Attribute List

Fix to Roslyn Style
Remove GetNextToken/GetPreviousToken

Add method to IsTypeAttributeContextBefore

Merging on behalf of @Neme12. Thanks!