LibJS: Rename Allocator => CellAllocator

Now that we have a BlockAllocator as well, it seems appropriate to name
the allocator-that-allocates-cells something more specific to match.

Userland/Libraries/LibJS/Forward.h

@@ -101,7 +101,7 @@
 namespace JS {
 
 class ASTNode;
-class Allocator;
+class CellAllocator;
 class BigInt;
 class BoundFunction;
 class Cell;