feat: esm + tshy

[kanadgupta]

🚥 Resolves ISSUE_ID

🧰 Changes

Spike on using tshy for the build — seems more strictly typed than tsup (see this tweet from a TS maintainer)

TODO

• Look into using ESNext / bundler in tsconfig for core subpackage (based on my reading, I think the moduleResolution is the one that needs extra revisiting)

Current blockers:

• Figure out how to properly load in the spec file — currently we're in a catch-22 where the JSON import isn't being properly asserted and then import assertions aren't supported in CJS
• @readme/api-core isn't a module (we really gotta merge in feat: migrate api and core to ESM #733)

🧬 QA & Testing

Provide as much information as you can on how to test what you've done.