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/LibGfx/Path.cpp

@@ -231,7 +231,7 @@ String Path::to_string() const
 
         builder.append(") "sv);
     }
-    builder.append("}");
+    builder.append('}');
     return builder.to_string();
 }