LibJS: Generate bytecode for entering nested lexical environments

This adds a new PushLexicalEnvironment instruction that creates a new
LexicalEnvironment and pushes it on the VM's scope stack.

There is no corresponding PopLexicalEnvironment instruction yet,
so this will behave incorrectly with let/const scopes for example.

Userland/Libraries/LibJS/Bytecode/Instruction.h

@@ -58,6 +58,7 @@
     O(Increment)                  \
     O(Decrement)                  \
     O(Throw)                      \
+    O(PushLexicalEnvironment)     \
     O(EnterUnwindContext)         \
     O(LeaveUnwindContext)         \
     O(ContinuePendingUnwind)