Generate .d.ts files for `unstable_core` module

[aabccd021]

Currently only index.d.ts and types.d.ts are output correctly and the rest of files export empty object.

I'm not familiar with rollup but let me know if I can help.

[inokawa]

Probably we have to do:

• Remove @ts-nocheck from lib/core/index.d.ts (and also check svelte adapter has no errors) https://github.com/inokawa/virtua/blob/5d5ef0700f779e3228d1eae62f101dfd3709d878/scripts/rollup-plugin-svelte-copy.mjs#L25
• Remove @internal annotations from external APIs, because they are stripped by tsc on publish https://www.typescriptlang.org/tsconfig/#stripInternal