feat: add Express.js example for Better Auth

[JayShende]

Summary

Adds a new Express.js example demonstrating how to use Better Auth with Prisma.

What’s included

• Express.js server setup
• Better Auth initialization
• Email/password authentication APIs
• Auth middleware for protected routes
• Detailed README for setup and usage

Closes #1

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}