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/Instruction.h

@@ -24,6 +24,7 @@
     O(BlockDeclarationInstantiation)   \
     O(Call)                            \
     O(CallWithArgumentArray)           \
+    O(Catch)                           \
     O(ConcatString)                    \
     O(ContinuePendingUnwind)           \
     O(CopyObjectExcludingProperties)   \