Use language settings' prettier parsers as a fallback for files with no path

[SomeoneToIgnore]

Follow-up of #12095 (comment) reverting back part of #11558 that was related to language.toml parsing.

Now all extensions that define prettier_parser_name in their language configs, will enable formatting untitled buffers without any extra language settings like

{
  "languages": {
    "JSON": {
      "prettier": {
        "allowed": true,
        "parser": "json"
      }
    }
  }
}

Release Notes:

• Improved ergonomics of untitled buffer formatting with prettier, no extra language settings are needed by default.