AK: Replace the mutable String::replace API with an immutable version

This removes the awkward String::replace API which was the only String
API which mutated the String and replaces it with a new immutable
version that returns a new String with the replacements applied. This
also fixes a couple of UAFs that were caused by the use of this API.

As an optimization an equivalent StringView::replace API was also added
to remove an unnecessary String allocations in the format of:
`String { view }.replace(...);`

AK/StringView.h

@@ -220,6 +220,7 @@ public:
 
     [[nodiscard]] bool is_whitespace() const { return StringUtils::is_whitespace(*this); }
 
+    [[nodiscard]] String replace(const StringView& needle, const StringView& replacement, bool all_occurrences = false) const;
     [[nodiscard]] size_t count(StringView const& needle) const { return StringUtils::count(*this, needle); }
 
     template<typename... Ts>