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/Utilities/diff.cpp

@@ -41,11 +41,11 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
 
         // Action
         if (num_added > 0 && num_removed > 0)
-            sb.append("c");
+            sb.append('c');
         else if (num_added > 0)
-            sb.append("a");
+            sb.append('a');
         else
-            sb.append("d");
+            sb.append('d');
 
         // Target line(s)
         sb.appendff("{}", target_start);