AK: Add StringBuilder::to_fly_string()

AK/StringBuilder.cpp

@@ -15,6 +15,7 @@
 
 #ifndef KERNEL
 #    include <AK/DeprecatedString.h>
+#    include <AK/FlyString.h>
 #    include <AK/Utf16View.h>
 #endif
 
@@ -122,6 +123,11 @@ ErrorOr<String> StringBuilder::to_string() const
 {
     return String::from_utf8(string_view());
 }
+
+ErrorOr<FlyString> StringBuilder::to_fly_string() const
+{
+    return FlyString::from_utf8(string_view());
+}
 #endif
 
 StringView StringBuilder::string_view() const

AK/StringBuilder.h

@@ -66,6 +66,7 @@ public:
 #endif
 
     ErrorOr<String> to_string() const;
+    ErrorOr<FlyString> to_fly_string() const;
 
     [[nodiscard]] ByteBuffer to_byte_buffer() const;