chore: stop tracking next-env.d.ts (#302)

## Summary
- stop tracking `apps/devkit/next-env.d.ts`
- add `next-env.d.ts` to the repository root `.gitignore`

## Testing
- not run (no runtime code change)

Author: kdy1

.gitignore

@@ -134,6 +134,7 @@ web_modules/
 # Next.js build output
 .next
 out
+next-env.d.ts
 
 # Nuxt.js build / generate output
 .nuxt