feat: add time_on_page trigger

[tyiuhc]

Summary

Support time_on_page page trigger, fired when the end user has been on the page for a specified durationMs.

Checklist

• Does your PR title have the correct title format?
• Does your PR have a breaking change?: No

[macroscopeapp]

Add time_on_page trigger and publish time_on_page events via SubscriptionManager.setUpTimeOnPagePublisher in subscriptions.ts

Introduce a time_on_page trigger with timers, initial publish, and activation checks; extend message payloads and PageObject types to support durationMs-based evaluation.

📍Where to Start

Start with SubscriptionManager.setUpTimeOnPagePublisher in subscriptions.ts.

Changes since #242 opened

• Removed clearTimeout invocation from SubscriptionManager class's setTimeout callback for time_on_page trigger scheduling [945bbbb]
• Refactored time_on_page trigger mechanism to use message payload-based duration tracking instead of internal state [91f88e1]

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Macroscope summarized c6ed9a4.

[promptless]

📝 Documentation updates detected!

Updated existing suggestion: Document new page trigger types for Web Experiment

[stephen-choi-amplitude]

1. It's missing visibilitychange event handler to reset when the user moves away from the current tab
2. Q: When there are multiple page objects with the time on page trigger type, are there any potential issues?

[tyiuhc]

1. should time on page be cumulative? Or should the trigger be reset to 0 if it has not already fired?
2. there should not be issues with this, but deduping of page object-triggered actions will be more formally addressed in another PR.

[stephen-choi-amplitude]

1. Per tech doc:

The timer resets whenever the user leaves the tab/window and returns

IOW, clear timeout when the current tab is hidden, and restart