Support `client/registerCapability` and `initialize` message `executeCommandProvider` under capabilities

[BoykoAlex]

LSP defines client/registerCapability request message to register capabilities such as workspace commands on the client (workspace/executeCommand).

[BoykoAlex]

@damieng is there a good place in AtomEnvironment to register workspace/executeCommand handlers? We found CommandRegistry but that registry seem to be UI/Command Palette specific... wasn't clear how to invoke the commands with parameters...

[BoykoAlex]

This seems to be important for JDT LS extensions. JDT LS extensions were introduced for vscode java debugger: https://github.com/Microsoft/vscode-java-debug

[UziTech]

Development of atom-languageclient has officially moved to https://github.com/atom-ide-community/atom-languageclient 🎉

If this is still an issue please consider opening an issue on that repo.