index.d.cts makes CommonJS TypeScript projects using module: node16 fail with TS1479 #850

A CommonJS TypeScript project that uses module: node16 and skipLibCheck: false cannot type-check an import of reactfire. I found this while reviewing #838, and it is already true on main.

What happens

• tsc reports TS1479 on every line of node_modules/reactfire/dist/index.d.cts that imports or re-exports a local file: "The current file is a CommonJS module whose imports will produce 'require' calls; however, the referenced file is an ECMAScript module and cannot be imported with 'require'."
• I got 11 of these on main and 12 on fix(build): preserve ESM modules so bundlers can tree-shake unused Firebase SDKs #838's head, where the extra one is the new ./utils.js re-export.
• With skipLibCheck: true the errors go away and the imported types still resolve correctly, so projects with that setting are not affected.
• attw reports all four resolution modes green, so the CI "Verify package" job does not catch it.

Steps to reproduce

• Create a project with no "type" field in its package.json and a tsconfig.json with module and moduleResolution set to node16, skipLibCheck: false.
• Install reactfire (I used a tarball packed from main at eb3376a), firebase 13.1.0 and React 19.
• Import useUser from reactfire in a .ts file and run tsc.

Cause, as far as I can tell

• The build:types-cjs script copies dist/index.d.ts to dist/index.d.cts, so the two files are identical.
• The .d.cts file is treated as CommonJS, but it re-exports from ./auth.js and the other per-module declarations, which sit in a "type": "module" package and are therefore ESM.

I have not tried a fix. If emitting real CommonJS declarations is more than this is worth, documenting skipLibCheck: true for CommonJS projects may be enough.