Allow disabling GA4_MEASUREMENT_ID not-set warning in non-prod env

[behnam-oneschema]

With this plugin enabled, every single reload of Deno Fresh throws a

GA4: GA4_MEASUREMENT_ID environment variable not set. Google Analytics reporting disabled.

warning on the console.

Considering that it is in fact recommended to not set GA4_MEASUREMENT_ID env var in the development environment, I think this plugin shouldn't be warning about this, or at least have an easy way to opt out.