feat: make RequestEvent readonly instead of frozen

[DustinJSilk]

What is it?

• Feature
• types

Description

Switch from freezing RequestEvents to using readonly so that users can extend the RequestEvent while keeping the event safe from malpractice internally.

Checklist

• [x] My code follows the developer guidelines of this project
• [x] I performed a self-review of my own code
• [x] I added a changeset with pnpm change
• [ ] I made corresponding changes to the Qwik docs
• [ ] I added new tests to cover the fix / functionality