After the previous PR we no longer depend on SWC for the critical path of
development/testing. Notably this unblocks adding support for the rest of the
language: the new `forget_hermes_parser` crate automatically converts Hermes
Parser's AST into our `forget_estree` format via codegen. Achieving full
language support with SWC would have required manually defining the remaining
conversions for the rest of the language.
Long-term we'll need to revisit how to integrate into SWC, and more generally
into setups build atop SWC such as Next.js's Turbopack-based build
configuration. But i'll delete for now since we don't depend on it for
iteration, it's slow to build, and requires us to opt-in to nightly Rust.