feat: add Cmd+, keyboard shortcut to toggle settings (#319)

Add settings.toggle action with Cmd+, (Mac) / Ctrl+, (non-Mac) bindings.
When on a settings route, pressing the shortcut navigates back; otherwise
it pushes to the host settings route. Also adds comma to the shortcut
key map so the combo string parses correctly.

packages/app/src/keyboard/shortcut-string.ts

@@ -31,6 +31,7 @@ KEY_MAP["Digit"] = { code: "Digit" };
 KEY_MAP["\\"] = { code: "Backslash" };
 KEY_MAP["["] = { code: "BracketLeft", key: "[" };
 KEY_MAP["]"] = { code: "BracketRight", key: "]" };
+KEY_MAP[","] = { code: "Comma", key: "," };
 KEY_MAP["."] = { code: "Period", key: "." };
 KEY_MAP["`"] = { code: "Backquote", key: "`" };
 KEY_MAP["/"] = { code: "Slash" };