Added key mappings for [, ], , and \. (#3373)

Added key mappings for '[', ']', ',' and '\'.

* Closes https://github.com/emilk/egui/issues/3372

## Added keys

* egui::Key::Backslash
* egui::Key::OpenBracket
* egui::Key::CloseBracket
* egui::Key::Comma

Co-authored-by: Mike Krüger <mkrueger@posteo.de>

crates/egui-winit/src/lib.rs

@@ -1066,6 +1066,10 @@ fn key_from_key_code(key: winit::keyboard::KeyCode) -> Option<egui::Key> {
         KeyCode::Period => Key::Period,
         // KeyCode::Colon => Key::Colon, // NOTE: there is no physical colon key on an american keyboard
         KeyCode::Semicolon => Key::Semicolon,
+        KeyCode::Backslash => Key::Backslash,
+        KeyCode::BracketLeft => Key::OpenBracket,
+        KeyCode::BracketRight => Key::CloseBracket,
+        KeyCode::Backquote => Key::Backtick,
 
         KeyCode::Cut => Key::Cut,
         KeyCode::Copy => Key::Copy,