Enable clippy::single_char_pattern (#8727)

This PR enables the
[`clippy::single_char_pattern`](https://rust-lang.github.io/rust-clippy/master/index.html#/single_char_pattern)
rule and fixes the outstanding violations.

Release Notes:

- N/A

crates/language/src/buffer_tests.rs

@@ -1110,7 +1110,7 @@ fn test_autoindent_does_not_adjust_lines_with_unchanged_suggestion(cx: &mut AppC
                 b();
                 |
             "
-            .replace("|", "") // marker to preserve trailing whitespace
+            .replace('|', "") // marker to preserve trailing whitespace
             .unindent(),
         )
         .with_language(Arc::new(rust_lang()), cx);
@@ -1787,7 +1787,7 @@ fn test_language_scope_at_with_javascript(cx: &mut AppContext) {
 
         // In a JSX expression: use the default config.
         let expression_in_element_config = snapshot
-            .language_scope_at(text.find("{").unwrap() + 1)
+            .language_scope_at(text.find('{').unwrap() + 1)
             .unwrap();
         assert_eq!(
             expression_in_element_config
@@ -2321,7 +2321,7 @@ fn test_trailing_whitespace_ranges(mut rng: StdRng) {
         actual_ranges,
         expected_ranges,
         "wrong ranges for text lines:\n{:?}",
-        text.split("\n").collect::<Vec<_>>()
+        text.split('\n').collect::<Vec<_>>()
     );
 }