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)

  virtua/scripts/rollup-plugin-svelte-copy.mjs

  Line 25 in 5d5ef07

  writeFileSync(coreDts, "// @ts-nocheck\n" + readFileSync(coreDts));

• Remove @internal annotations from external APIs, because they are stripped by tsc on publish https://www.typescriptlang.org/tsconfig/#stripInternal