LibJS/JIT: Update "unwind context" stack in JIT code

Until now, the unwind context stack has not been maintained by jitted
code, which meant we were unable to support the `with` statement.
This is a first step towards supporting that by making jitted code
call out to C++ to update the unwind context stack when entering/leaving
unwind contexts.

We also introduce a new "Catch" bytecode instruction that moves the
current exception into the accumulator. It's always emitted at the start
of a "catch" block.

Userland/Libraries/LibJS/Bytecode/Interpreter.h

@@ -74,6 +74,7 @@ public:
 
     void enter_unwind_context();
     void leave_unwind_context();
+    void catch_exception();
 
     Executable& current_executable() { return *m_current_executable; }
     Executable const& current_executable() const { return *m_current_executable; }