Trigger signature_help on comma (#564)

Fixes an issue where some tools are not properly triggered for signature
help. See ray-x/lsp_signature.nvim#38

Author: jared-mackey

apps/language_server/lib/language_server/providers/signature_help.ex

@@ -1,7 +1,7 @@
 defmodule ElixirLS.LanguageServer.Providers.SignatureHelp do
   alias ElixirLS.LanguageServer.SourceFile
 
-  def trigger_characters(), do: ["("]
+  def trigger_characters(), do: ["(", ","]
 
   def signature(%SourceFile{} = source_file, line, character) do
     response =