LibJS: Implement 'this' in the bytecode VM

ThisExpression now emits a "ResolveThisBinding" bytecode op, which
simply loads the VM's current 'this' binding into the accumulator.

Userland/Libraries/LibJS/Bytecode/Instruction.h

@@ -61,6 +61,7 @@
     O(PushDeclarativeEnvironment)    \
     O(PutById)                       \
     O(PutByValue)                    \
+    O(ResolveThisBinding)            \
     O(Return)                        \
     O(RightShift)                    \
     O(SetVariable)                   \