Put Symbols in the constant.language namespace

[OmriSama]

This change puts Symbols in constant.language Textmate namespace/scope, which gives it special highlighting likening it to the Ruby extension. Before/After with Material Community Theme (Dark High Contranst):

Before

After

[axelson]

Thanks! I think this is a good change, but we should update the rest of the atom definitions to be constant.language.symbol.elixir as well, so I've added a commit to update them.