Introduce config options for Turbopack GC (#98378)
### What?
Replaces the Turbopack GC environment variables with an
`experimental.turbopackGc`
config option
```js
// next.config.js
experimental: {
turbopackGc: true, // on, defaults
turbopackGc: { minProgressMs: 100, rootTtlMs: 432e6 }, // on, tuned
}
```
This replaces the env vars which were unused and not the ergonomic
