LibJS: Collapse handler/finalizer into single exception handler target

After replacing the runtime unwind context stack with explicit
completion records for try/finally dispatch, the distinction between
"handler" (catch) and "finalizer" (finally) in the exception handler
table is no longer meaningful at runtime.

handle_exception() checked handler first, then finalizer, but they
did the exact same thing (set the PC). When both were present, the
finalizer was dead code.

Collapse both fields into a single handler_offset (now non-optional,
since an entry always has a target), remove the finalizer concept
from BasicBlock, UnwindContext, and ExceptionHandlers, and simplify
handle_exception() to a direct assignment.

Libraries/LibJS/Bytecode/Executable.cpp

@@ -81,11 +81,10 @@ void Executable::dump() const
         warnln("");
         warnln("Exception handlers:");
         for (auto& handlers : exception_handlers) {
-            warnln("    from {:4x} to {:4x} handler {:4x} finalizer {:4x}",
+            warnln("    from {:4x} to {:4x} handler {:4x}",
                 handlers.start_offset,
                 handlers.end_offset,
-                handlers.handler_offset.value_or(0),
-                handlers.finalizer_offset.value_or(0));
+                handlers.handler_offset);
         }
     }