doc: guide for runtime-specific conditional exports using --import

[Momena-akhtar]

Description

This PR adds an example to the doc/api/esm.md documentation demonstrating how to simulate runtime-specific conditional exports (such as "electron") using the --import flag.

Since Node.js does not currently support dynamically registering custom conditions for "exports" resolution, this guide shows a practical workaround that leverages environment variables and preload scripts.

The example illustrates:

• Creating a preload script to detect the runtime (e.g., Electron)
• Conditionally exporting different modules at runtime
• Limitations and use cases

This replaces the previous proposal to add a registerCondition() API by showing a userland solution that does not require changes to Node.js core.

✳️ Moved from the previous location (doc/contributing/) to the API documentation under doc/api/esm.md as recommended in #59066

[nodejs-github-bot]

Review requested:

• @nodejs/loaders