Call `register_assignment_scope()` in `visit_export_default_expr()` and `visit_export_default_decl()` (#94581)
Talked IRL, we found that calling this in `visit_export_default_expr()`
cut down the `module-cost` benchmark on ESM execution by about 10ms
(approx. 35%).
Doing this unlocks a codegen optimization to avoid arrow functions, this
special case was needed to support `export default ...` which is a kind
of declaration all on its own.