Improve tab event sourcing

[hymccord]

This PR depends on #69 being accepted

• feat(tab): use source window as ref to send handshake instead of setting a window var

Fixes: #15

[Copilot]

Pull Request Overview

This PR improves tab event sourcing by replacing the global window.newlyTabOpened variable approach with a more reliable method that uses the source window reference to identify tabs during handshake operations.

• Removes global window.newlyTabOpened variable from tab creation and event handling
• Enhances tab identification by using message source and searching tabs by ID or window reference
• Updates function signatures to accept source parameter for better tab resolution

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/tab.js	Removes setting of global window.newlyTabOpened variable during tab creation
src/event-listeners/postmessage.js	Refactors event handlers to use source window reference instead of global variable for tab identification

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