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/JIT/Compiler.h

@@ -83,6 +83,7 @@ private:
         O(EnterUnwindContext, enter_unwind_context)                              \
         O(LeaveUnwindContext, leave_unwind_context)                              \
         O(Throw, throw)                                                          \
+        O(Catch, catch)                                                          \
         O(CreateLexicalEnvironment, create_lexical_environment)                  \
         O(LeaveLexicalEnvironment, leave_lexical_environment)                    \
         O(ToNumeric, to_numeric)                                                 \