new_audit(best-practices): add React development mode audit

[plutoTaco]

Summary
This introduces a new_audit to the Best Practices category.

React development builds are unoptimized and significantly larger than production builds, which negatively impacts page load performance. This PR introduces a new Gatherer and Audit to detect if a target page is currently running an unoptimized React development build.

• Gatherer (react-dev-mode.js): Utilizes Page.addScriptToEvaluateOnNewDocument during the startInstrumentation lifecycle to safely inject a dummy __REACT_DEVTOOLS_GLOBAL_HOOK__. This intercepts the React renderer upon boot to evaluate bundleType before the page fully loads, bypassing NO_FCP timeouts.
• Audit (react-dev-mode.js): Scores a 0 if a development build is detected, and 1 if running in production (or if React is not present).

Related Issues/PRs

• Added the gatherer and audit to core/config/default-config.js as required.
• Included unit tests via Mocha in core/test/audits/react-dev-mode-test.js.
• Formatted the Audit description to explain both importance and remediation, as an official reference doc does not exist yet.

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