Add promise client APIs with callback deprecations

[ricardo-devis-agullo]

Summary

• add promise forms for client initialization, component rendering, component info, and template rendering
• preserve legacy callback timing, return values, error identity, and multi-value callback results
• emit one process-wide deprecation warning for callback usage and expose an explicit promise API capability marker
• document promise resolution and rejection behavior

Coordinates with opencomponents/oc#1550.

Test plan

• volta run --node 24.15.0 npx mocha test/unit/client.js (7 passing)
• npx prettier-eslint src/index.js test/unit/client.js --list-different
• volta run --node 24.15.0 npm test (97 passing; 6 pre-existing Node 24/minimal-request failures)

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