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/file_finder/src/file_finder.rs

@@ -701,7 +701,7 @@ impl PickerDelegate for FileFinderDelegate {
         raw_query: String,
         cx: &mut ViewContext<Picker<Self>>,
     ) -> Task<()> {
-        let raw_query = raw_query.replace(" ", "");
+        let raw_query = raw_query.replace(' ', "");
         let raw_query = raw_query.trim();
         if raw_query.is_empty() {
             let project = self.project.read(cx);