Stop the XMLServerLauncher process when the extension is deactivated

[xorye]

Sometimes when you exit VS Code while vscode-xml was running, there is a Java process called XMLServerLauncher that remains.

If that process remains, and vscode-xml is started up again, there is an error about the XML language server crashing 5 times in the last 3 minutes.

To fix this problem vscode-xml should call stop() on the languageClient in the deactivate() method, which is what vscode-java does:
https://github.com/redhat-developer/vscode-java/blob/c7afc32bde70f5afc292966c1311ec4dd1bfe9b5/src/extension.ts#L384-L387

to ensure that the process is terminated when the extension is deactivated.