Improve Elixir support

[leandrocp]

Migrate LiveBook's Elixir cell_editor configuration over to live_monaco_editor

• intellisense
• formatting
• signature help

Note it's important to maintain the proper credits and attribution as done at

live_monaco_editor/assets/js/live_monaco_editor/editor/code_editor.js

Lines 1 to 3 in 046f263

	// Copied and modified from the original work available at https://github.com/livebook-dev/livebook/blob/8532bc334bdcf3c57fab9b694666e609877d279f/assets/js/hooks/cell_editor/live_editor.js
	// Copyright (C) 2021 Dashbit
	// Licensed under Apache 2.0 available at https://www.apache.org/licenses/LICENSE-2.0