New Lint rule to restrict process env usage

[fichtnerma]

Description

Added new eslint rules to restrict usage of process.env

• Restrict using NEXT_PUBLIC_ variables
• Restrict using other env variables than NODE_ENV in next.config.js
• Restrict using process.env in *.loader.ts files

Acceptance criteria

• I have verified if my change requires an example
• I have verified if my change requires a changeset
• I have verified if my change requires screenshots/screencasts

Further information

• Task: https://vivid-planet.atlassian.net/browse/COM-1290

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[nsams]

@fraxachun loader.ts is just a convention: the file can have any name or the loader function can be defined in the component file

do you think this catches a common error, although not 100%?

[nsams]

for site-pages this rule should be disabled for the whole site. And you should mention in the changeset (or even migration guide) that for projects using the old build that this rule should also be disabled there.

[johnnyomair]

IMO we shouldn't add this to the package at all, but make it a Demo/Starter-only change

Reasons:

1. It's highly specific for our deployment setup
2. The file name patterns will change often (e.g., it's next.config.mjs in the Starter, and will be next.config.ts starting with Next 15)
3. There's no simple way to disable just this rule, devs would have to disable "no-restricted-syntax" altogether. This is okay for now, but once we add another rule, it will be disabled as a side effect.