[Excel] (Chart) Add leader lines snippet

[alison-mk]

This PR is in draft mode because the enableLeaderLines method isn't working properly yet -- it's adding leader lines, which shouldn't happen until the enableLeaderLines method.

[Copilot]

Pull Request Overview

This PR adds a new Excel chart leader lines snippet that demonstrates how to show, hide, and format leader lines for chart data labels. The sample provides a complete workflow for working with leader lines in Excel charts using Office.js APIs.

• Adds a new sample file demonstrating leader lines functionality for Excel charts
• Implements functions to enable/disable leader lines and modify their formatting
• Provides UI controls for testing all leader line operations

[Copilot]

The comment suggests loading the current setting, but the loaded value is never used in the function. Consider removing the load operation and comment, or clarify why the current value needs to be loaded.

Suggested change

	// Load the current leader lines setting.
	dataLabels.load("showLeaderLines");
	await context.sync();

[Copilot]

The showLeaderLines property is loaded but never accessed before being set to true. This load operation appears unnecessary and could be removed for better performance.

Suggested change

	// Load the current leader lines setting.
	dataLabels.load("showLeaderLines");
	await context.sync();
	// Enable leader lines.
	dataLabels.showLeaderLines = true;
	// Enable leader lines.
	dataLabels.showLeaderLines = true;
	await context.sync();