Implement invalidateOnEnvChange

[sailerinteractive]

I just had a discussion about invalidating the transformer cache when an environment variable has changed.
See parcel-bundler/parcel#8573

I would like to implement asset.invalidateOnEnvChange() for parcel-transformer-ejs but im not sure about the best way to build a list of all used in env vars in a template. Any thoughts on this?

[sarscode]

Hi @sailerinteractive, great idea!!! We can do this by supporting env var with prefix PARCEL_EJS_*. Feel free to submit a PR for this if you already have an implementation.

[sailerinteractive]

As far as i understand ENV vars are always addressed through "process.env.VAR_NAME" My approach would be to simple run a regular expression for that on the template and then iterate through the results and call asset.invalidateOnEnvChange(...) Does that make sense?