Ensure app config variables have values at build-time

[art-alexeyenko]

This PR slightly reworks how a config in nextjs app is built.
Each value is resolved with the following logic:

1. Attempt to retrieve value from env
2. Attempt to retrieve value from supplementary files (package.json / scjssconfig)
3. If certain values (that can't be empty, i.e. language) are still empty - use fallback value.

This roughly follows pre-plugin approach to building config where env values take precedence, other sources follow as fallback.
This also ensures that graphQL endpoint will have a correct value, if hostname values is present in either env or scjssconfig.

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)

[ambrauer]

The computed prop and computeConfigValue required to evaluate the string seems unnecessary. I think you're onto something with the fallback to env values in scjssconfig.ts...

Could we just make this a first step to populate all of the config defaults?
For example, in generate-config.ts, something like:

const defaultConfig: JssConfig = {
  sitecoreApiKey: process.env[`${constantCase('sitecoreApiKey')}`] || 'no-api-key-set',
  sitecoreApiHost: process.env[`${constantCase('sitecoreApiHost')}`] || '',
  jssAppName: process.env[`${constantCase('jssAppName')}`] || 'Unknown',
  graphQLEndpointPath: process.env[`${constantCase('graphQLEndpointPath')}`] || '',
  defaultLanguage: process.env[`${constantCase('defaultLanguage')}`] || 'en',
};

Or maybe even better, a default config plugin that does this? Could probably then lose the defaultConfig altogether in generate-config.ts.

[art-alexeyenko]

Reworked the approach, moving away from dynamic evaluation.

[ambrauer]

Looking good, see a few comments

[ambrauer]

Looks good!

[ambrauer]

Resolves #1291