[templates/nextjs] Add .env.* support

[addy-pathania]

• This PR follows the Contribution Guide
• Changelog updated

Description / Motivation

Provide built-in support for environment variables which Nextjs provides OOTB i.e use .env.* files to load environment variables.
Refer to : Nextjs environment variables support for more details.

Testing Details

• Unit Test Added
• Manual Test/Other (Please elaborate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

[yavorsk]

Looks good to me, see one comment and maybe PR description should be in its section

[illiakovalenko]

1. I think we shouldn't use nextjs env package for this purpose, since these scripts are just node.js scripts, and mainly this package is recommended to be used for unit tests.
   Please, take a look at other packages, like dotenv-flow, I think it was mentioned in a task description
2. If I start the app in production mode or connected, how the "loadEnvConfig" will automatically identify which .env file needs to be loaded? Probably, you need to set up NODE_ENV env variable, from my understanding

[addy-pathania]

@illiakovalenko

1. I think we shouldn't use nextjs env package for this purpose, since these scripts are just node.js scripts, and mainly this package is recommended to be used for unit tests.
   Please, take a look at other packages, like dotenv-flow, I think it was mentioned in a task description
2. If I start the app in production mode or connected, how the "loadEnvConfig" will automatically identify which .env file needs to be loaded? Probably, you need to set up NODE_ENV env variable, from my understanding

1. I don't think it's mentioned anywhere that it should only be used for unit test purpose. We can load this for any nodejs environment. Also, we mentioned to look into this solution as well in our grooming notes. From my understanding it's better to use this rather than installing another dependency.
2. This will be the priority order for loading of env files. Yes they would have to set the NODE_ENV if they want to use the .env.$(NODE_ENV) file but for me it worked without setting NODE_ENV.

[illiakovalenko]

@sc-addypathania
1- it's mentioned here
https://nextjs.org/docs/pages/building-your-application/configuring/environment-variables#test-environment-variables
or it's commonly used still?

2 - if you start your app:

• jss:start:connected .env.development should be pulled
• jss start:production .env.production should be pulled
  is it working?

[ambrauer]

👍

[addy-pathania]

@sc-addypathania 1- it's mentioned here https://nextjs.org/docs/pages/building-your-application/configuring/environment-variables#test-environment-variables or it's commonly used still? 2 - if you start your app:

• jss:start:connected .env.development should be pulled
• jss start:production .env.production should be pulled
  is it working?

@illiakovalenko FYI these comments should be addressed now with the dotenv-flow package and adding NODE_ENV to the package.json scripts.