AK: Move data fields from AK::String to a newly created AK::StringBase

This starts separating memory management of string data and string
utilities like `String::formatted`. This would also allow to reuse the
same storage in `DeprecatedString` in the future.

AK/CMakeLists.txt

@@ -28,6 +28,7 @@ set(AK_SOURCES
     StackInfo.cpp
     Stream.cpp
     String.cpp
+    StringBase.cpp
     StringBuilder.cpp
     StringFloatingPointConversions.cpp
     StringImpl.cpp