[plug-in][languages] Add ability to register completion provider to Plugin API

[ashumilova]

In the languages namespace there should be a way to register a completion provider for a language and point trigger completion item (the symbol which will trigger the completion start - '.', ',') as well.
The completion provider defines the contract between extensions and IntelliSense (variety of code editing features including: code completion, parameter info, quick info, and member lists)

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