AK+Everywhere: Rename String to DeprecatedString

We have a new, improved string type coming up in AK (OOM aware, no null
state), and while it's going to use UTF-8, the name UTF8String is a
mouthful - so let's free up the String name by renaming the existing
class.
Making the old one have an annoying name will hopefully also help with
quick adoption :^)

AK/CMakeLists.txt

@@ -1,6 +1,7 @@
 set(AK_SOURCES
     Assertions.cpp
     Base64.cpp
+    DeprecatedString.cpp
     FloatingPointStringConversions.cpp
     FlyString.cpp
     Format.cpp
@@ -14,7 +15,6 @@ set(AK_SOURCES
     LexicalPath.cpp
     Random.cpp
     StackInfo.cpp
-    String.cpp
     StringBuilder.cpp
     StringFloatingPointConversions.cpp
     StringImpl.cpp