Update Next.js example to v13

[rafaelalmeidatk]

Goal

👋 Hi there, I recently added Bugsnag to a project and noticed the example is using a very old version of Next.js (from 2021!) so I went ahead and updated some of the files. Since I kept the pages dir I didn't have to change too much stuff, most of the changes are to simplify the code

Changeset

• Updated all dependencies (I worked with a fresh lockfile so that's why the diff is large)
• Added type-checking to the next.config.js file
• Changed hardcoded internal strings with exported variables from next/constants
• Removed unnecessary NEXT_IS_SERVER env variable
• Removed usage of IgnorePlugin from the custom webpack config. Next.js does server-side code elimination so this library isn't bundled to the client

Testing

I tested the updated version with a new project in the Bugsnag dashboard

[johnkiely1]

Hey @rafaelalmeidatk, thanks for this PR, we will review as soon as priorities allow.