LibJS: Add a separate "identifier table" to bytecode executables

This is a specialized string table for storing identifiers only.
Identifiers are always FlyStrings, which makes many common operations
faster by allowing O(1) comparison.

Userland/Libraries/LibJS/Bytecode/Executable.cpp

@@ -17,6 +17,10 @@ void Executable::dump() const
         outln();
         string_table->dump();
     }
+    if (!identifier_table->is_empty()) {
+        outln();
+        identifier_table->dump();
+    }
 }
 
 }