feat: introduce env var to include dev server plugin

[davbree]

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Summary

Fixes #<replace_with_issue_number>

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[github-actions]

📊 Benchmark results

Comparing with de0942a

• Dependency count: 1,295 (no change)
• Package size: 284 MB ⬇️ 0.00% decrease vs. de0942a
• Number of ts-expect-error directives: 1,180 (no change)

[eduardoboucas]

Is this the customer-facing name we're going with? And is there value in showing this message? I believe we'll still show a generic message from Netlify Build indicating that this plugin is being loaded? I would consider printing this only if debug is true.