chore(@sentry/nextjs): move webpack-plugin to devDeps

[belgattitude]

Trying out one option to fix #3865.

This is WIP/ Do Not Merge

• If you've added code that should be tested, please add tests.
• Ensure your code lints and the test suite passes (yarn lint) & (yarn test).

[kamilogorek]

That makes sense nonetheless. Thanks

[lobsterkatie]

Actually, it doesn't. We use the webpack plugin when an SDK user builds their app, so we need to include it in our package. I'll revert this.