Use language settings' prettier parsers as a fallback for files with no path (#12273)

Follow-up of
https://github.com/zed-industries/zed/pull/12095#issuecomment-2123230762
reverting back part of https://github.com/zed-industries/zed/pull/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

```json
{
  "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.

crates/language/src/language.rs

@@ -610,6 +610,10 @@ pub struct LanguageConfig {
     /// How to soft-wrap long lines of text.
     #[serde(default)]
     pub soft_wrap: Option<SoftWrap>,
+    /// The name of a Prettier parser that will be used for this language when no file path is available.
+    /// If there's a parser name in the language settings, that will be used instead.
+    #[serde(default)]
+    pub prettier_parser_name: Option<String>,
 }
 
 #[derive(Clone, Debug, Serialize, Deserialize, Default, JsonSchema)]
@@ -692,9 +696,10 @@ impl Default for LanguageConfig {
             overrides: Default::default(),
             word_characters: Default::default(),
             collapsed_placeholder: Default::default(),
-            hard_tabs: Default::default(),
-            tab_size: Default::default(),
-            soft_wrap: Default::default(),
+            hard_tabs: None,
+            tab_size: None,
+            soft_wrap: None,
+            prettier_parser_name: None,
         }
     }
 }
@@ -1372,6 +1377,10 @@ impl Language {
             language_name => language_name.to_lowercase(),
         }
     }
+
+    pub fn prettier_parser_name(&self) -> Option<&str> {
+        self.config.prettier_parser_name.as_deref()
+    }
 }
 
 impl LanguageScope {