Add trace config option

[dbernheisel]

This adds the "elixir.trace.server" option

After enabling and using some of the LS features, this should give some details in the output channel. For example, I saw this:

[Trace - 1:38:53 AM] Sending notification 'initialized'.
Params: {}

[Trace - 1:38:53 AM] Sending notification 'workspace/didChangeConfiguration'.
Params: {
    "settings": {
        "elixirLS": {
            "dialyzerEnabled": false,
            "dialyzerWarnOpts": [],
            "dialyzerFormat": "dialyzer",
            "mixEnv": "test",
            "fetchDeps": true,
            "suggestSpecs": true,
            "trace": {
                "server": "off"
            }
        }
    }
}

Mirrors from vscode extension: https://github.com/elixir-lsp/vscode-elixir-ls/pull/10