Centralize extension logging messages to an OutputChannel so users can select "ElixirLS" in the "Output" panel

When determining the status of any language server, the first place I check is the "Output" panel at the bottom of VS Code. Most LSes have a tab in the dropdown where their LS stdout is piped, along with any error messages. I believe this is the [`OutputChannel`](https://code.visualstudio.com/api/references/vscode-api#OutputChannel) API within VS Code.

Elixir LS does not seem to implement this API. Could it? It would make it easier to debug when things aren't working correctly.