Improve elixir grammar: function calls without parenthesis

[thiagomajesk]

As a follow up on #22: This new issue is meant to track progress on expanding the entity.name.function-call.elixir scope added on https://github.com/elixir-lsp/vscode-elixir-ls/pull/40 to function calls without parenthesis. As exemplified here, while we can't disambiguate calls for niladic functions, we probably could do it for functions that have at least one parameter.

Example: The current behavior for greet/1 is that greet("Hello") will be scoped to entity.name.function-call.elixir while greet "Hello" will be scoped to source.elixir.