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/Font/BitmapFont.cpp

@@ -356,7 +356,7 @@ String BitmapFont::variant() const
         if (builder.string_view() == "Regular"sv)
             builder.clear();
         else
-            builder.append(" ");
+            builder.append(' ');
         builder.append(slope_to_name(slope()));
     }
     return builder.to_string();