fix: emit declaration at package types path

[Pugsin]

Summary

• set the TypeScript plugin rootDir to guest-js
• make declaration output land at dist-js/index.d.ts, matching the package types and export metadata

Why

The published @choochmeque/tauri-plugin-sharekit-api@0.4.0-rc.3 tarball contains dist-js/guest-js/index.d.ts, but package metadata points to dist-js/index.d.ts. Consumers resolving the advertised types path get a missing file.

Validation

• npm pack @choochmeque/tauri-plugin-sharekit-api@0.4.0-rc.3 --silent confirmed package/dist-js/index.d.ts is absent and package/dist-js/guest-js/index.d.ts is present
• corepack pnpm install --frozen-lockfile --ignore-scripts
• corepack pnpm build
• verified dist-js/index.d.ts is emitted
• git diff --check