Add typo detection to CI (#4107)

Adding the typos crate to our CI will take some doing, as we have
several tests which rely on typos in various ways (e.g. checking state
as the user types), but I thought I'd take a first stab at fixing what
it finds.

Release Notes:

- N/A

crates/editor/src/editor.rs

@@ -8533,7 +8533,7 @@ impl Editor {
     ) {
         match event {
             multi_buffer::Event::Edited {
-                sigleton_buffer_edited,
+                singleton_buffer_edited,
             } => {
                 self.refresh_active_diagnostics(cx);
                 self.refresh_code_actions(cx);
@@ -8543,7 +8543,7 @@ impl Editor {
                 cx.emit(EditorEvent::BufferEdited);
                 cx.emit(SearchEvent::MatchesInvalidated);
 
-                if *sigleton_buffer_edited {
+                if *singleton_buffer_edited {
                     if let Some(project) = &self.project {
                         let project = project.read(cx);
                         let languages_affected = multibuffer