Everywhere: Replace single-char StringView op. arguments with chars

This prevents us from needing a sv suffix, and potentially reduces the
need to run generic code for a single character (as contains,
starts_with, ends_with etc. for a char will be just a length and
equality check).

No functional changes.

Userland/Libraries/LibGUI/Toolbar.cpp

@@ -61,9 +61,9 @@ private:
         StringBuilder builder;
         builder.append(action.text());
         if (action.shortcut().is_valid()) {
-            builder.append(" (");
+            builder.append(" ("sv);
             builder.append(action.shortcut().to_string());
-            builder.append(")");
+            builder.append(')');
         }
         return builder.to_string();
     }