snippets: Read user settings for workspace configuration (#13356)

Fixes #13334

Use `settings` field in `lsp` subsettings to fix up the settings as
wish:
```
  "lsp": {
    "snippet-completion-server": {
      "settings": {
        "max_completion_items": 20, 
        "snippets_first": false,
        "feature_words": true,
        "feature_snippets": true,
        "feature_paths": true,
        "feature_unicode_input": false
      }
    }
  }
```


Release Notes:

- N/A