(maint) Add client message constants

This commit adds the client message type constants as they were missing.

Author: glennsarti

server/lib/languageserver/constants.rb

@@ -49,4 +49,9 @@ module LanguageServer
   DIAGNOSTICSEVERITY_WARNING = 2
   DIAGNOSTICSEVERITY_INFORMATION = 3
   DIAGNOSTICSEVERITY_HINT = 4
+
+  MESSAGE_TYPE_ERROR = 1
+  MESSAGE_TYPE_WARNING = 2
+  MESSAGE_TYPE_INFO = 3
+  MESSAGE_TYPE_LOG = 2
 end