Create static JSON config file for standalone server output

[iiroj]

What?

Instead of inlining the build-time JSON config to the server.js file generated to the standalone output of Next.js build (with config option output: 'standalone', this PR changes the the build process to emit a separate JSON file which is required into the server.js file.

Why?

It's cleaner to separate the config into its own file, since it's static JSON anyway. This change also makes it easier for one to create their own server.js file.

How?

Instead of inlining the JSON.stringify(nextConfig) content into the generated server.js file, write a separate file next.config.json next to it, and use the already-created require to load it. This should work with both CommonJS and ESM since the createRequire is already configured. The require automatically parses JSON.

I did not add new tests because this should not change existing behavior, and there's plenty of existing tests for the standalone build output; and if this was broken, hopefully some of them would fail.

[ijjk]

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[iiroj]

The test production/standalone-mode/required-server-files/required-server-files.test.ts contains three assertions for the configuration content, which need to be updated to read the JSON instead:

    ✕ `compress` should be `false` in nextEnv (200 ms)
    ✕ `cacheHandler` should have correct path (194 ms)
    ✕ `cacheMaxMemorySize` should be disabled by setting to 0 (191 ms)

EDIT: ✅ updated the tests