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.

AK/MACAddress.h

@@ -75,7 +75,7 @@ public:
         if (string.is_null())
             return {};
 
-        auto const parts = string.split_view(":");
+        auto const parts = string.split_view(':');
         if (parts.size() != 6)
             return {};