Fix signature trigger characters (#654)

Fixes #651.

Tested in VS Code, works the same.

Author: jakebailey

src/LanguageServer/Impl/Implementation/Server.cs

@@ -81,7 +81,7 @@ public Server(IServiceManager services) {
                     triggerCharacters = new[] { "." }
                 },
                 hoverProvider = true,
-                signatureHelpProvider = new SignatureHelpOptions { triggerCharacters = new[] { "(,)" } },
+                signatureHelpProvider = new SignatureHelpOptions { triggerCharacters = new[] { "(", ",", ")" } },
                 definitionProvider = true,
                 referencesProvider = true,
                 workspaceSymbolProvider = true,