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/CMakeLists.txt

@@ -4,6 +4,7 @@ set(SOURCES
     Bytecode/BasicBlock.cpp
     Bytecode/Executable.cpp
     Bytecode/Generator.cpp
+    Bytecode/IdentifierTable.cpp
     Bytecode/Instruction.cpp
     Bytecode/Interpreter.cpp
     Bytecode/Op.cpp