AK: Use an enum instead of a bool for String::replace(all_occurences)

This commit has no behavior changes.

In particular, this does not fix any of the wrong uses of the previous
default parameter (which used to be 'false', meaning "only replace the
first occurence in the string"). It simply replaces the default uses by
String::replace(..., ReplaceMode::FirstOnly), leaving them incorrect.

AK/StringView.h

@@ -268,7 +268,7 @@ public:
     }
 
 #ifndef KERNEL
-    [[nodiscard]] String replace(StringView needle, StringView replacement, bool all_occurrences = false) const;
+    [[nodiscard]] String replace(StringView needle, StringView replacement, ReplaceMode) const;
 #endif
     [[nodiscard]] size_t count(StringView needle) const
     {