Add Slack/Discord alerting and integrate controls.alert() with DLQ replay

[Prateek32177]

Motivation

• Introduce a simple alerting experience for Slack and Discord to notify on webhook events and DLQ occurrences.
• Wire alerting into the Upstash controls API so callers can send alerts and optionally attempt DLQ replay in one call.
• Provide DX-friendly defaults and payload builders so alerts require minimal configuration while allowing overrides.

Description

• Added a new notifications module with types, utilities, constants, builders for Slack and Discord, and sendAlert to POST payloads to webhooks (src/notifications/*).
• Exposed notification internals via src/alerts.ts and re-exported alerts from the main entry (src/index.ts) for backward compatibility (__alertInternals alias).
• Implemented controls.alert() in src/upstash/controls.ts which accepts ControlAlertOptions, optionally runs replay(dlqId) when dlq: true, and calls sendAlert using TernControlsConfig.notifications.
• Extended Upstash types to include notifications config, ControlAlertOptions, and the TernControls interface, and updated exports (src/upstash/types.ts, src/upstash/index.ts).
• Updated README.md with usage examples and behavioral notes for controls.alert() and DLQ replay flow.

Testing

• Ran the test suite with npm test and the tests completed successfully.
• Ran platform tests with npm run test:all and they completed successfully.

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Codex Task