Fix more failure cases of assistant edits (#19653)

* Make `description` optional (since we describe it as optional in the
prompt, and we're currently not showing it)
* Fix fuzzy location bug that neglected the cost of deleting prefixes of
the query.
* Make auto-indent work for single-line edits. Previously, auto-indent
would not occur when overwriting a single line (without inserting or
deleting a newline)

Release Notes:

- N/A

crates/languages/src/lib.rs

@@ -288,15 +288,6 @@ fn load_config(name: &str) -> LanguageConfig {
         .with_context(|| format!("failed to load config.toml for language {name:?}"))
         .unwrap();
 
-    #[cfg(not(feature = "load-grammars"))]
-    {
-        config = LanguageConfig {
-            name: config.name,
-            matcher: config.matcher,
-            ..Default::default()
-        }
-    }
-
     config
 }