[2.0] TypeScript definitions are incorrect

**Describe the bug**
In almost all cases, the TypeScript declarations are incorrect. The mistake made is that you should actually have 2 declarations files per dual-emitted-module. E.g. `index.d.ts` and `index.d.mts`

You can check for yourself here https://arethetypeswrong.github.io/?p=dayjs%402.0.0-alpha.4

Simple reproduction: https://stackblitz.com/edit/vitejs-vite-n5nsvh?file=src%2Fmain.ts

**Expected behavior**
The TypeScript definitions are correct.

**Information**
- Day.js Version: 2.0.0-alpha.4

Start with the arethetypeswrong.org report and the StackBlitz reproduction at src/main.ts to confirm the incorrect declaration behavior for Day.js 2.0.0-alpha.4. Trace the package's published TypeScript declarations and module exports, then verify that dual-emitted modules provide the expected declaration files and that the reproduction type-checks correctly.

Written by the indexing model from the issue text.