Add ImplementationProvider Support

[mjbvz]

With microsoft/vscode#18346, we added a new ImplementationProviderto the VSCode api to support go to implementation scenarios. Support for this should also be exposed in the language server protocol

[dbaeumer]

Proposed protocol spec has been added to the node implementation and the protocol.

[dbaeumer]

See microsoft/vscode-languageserver-node#291

[rcjsuen]

@dbaeumer Okay to close since this got merged in with 39b1181?

[dbaeumer]

Closing. @rcjsuen thanks for reminding.