LibDiff: Add support for writing formatted context hunks

There is a little bit more complexity involved here than the other
formats. In particular, this is due to the need to determine whether
an addition line or removal line is just that, or a 'change'.

Userland/Libraries/LibDiff/Hunks.h

@@ -31,6 +31,8 @@ struct Line {
         Removal = '-',
         Context = ' ',
 
+        // NOTE: This should only be used when deconstructing a hunk into old and new lines (context format)
+        Change = '!',
     };
 
     static constexpr Operation operation_from_symbol(char symbol)