LibJS: Implement bytecode generation for UpdateExpression :^)

Added Increment and Decrement bytecode ops to support this. Postfix
updates use a temporary register to preserve the original value.

Note that this patch only implements Identifier updates. Member
expression updates are a TODO.

Userland/Libraries/LibJS/Bytecode/Instruction.h

@@ -54,7 +54,9 @@
     O(UnsignedRightShift)         \
     O(In)                         \
     O(InstanceOf)                 \
-    O(ConcatString)
+    O(ConcatString)               \
+    O(Increment)                  \
+    O(Decrement)
 
 namespace JS::Bytecode {