fix: render lazy charts already in viewport on page load

[HardeepAsrani]

Summary

The scroll listener for lazy charts never fired if the chart was already visible on page load, causing it to remain blank until the user scrolled. Extracted the logic into a named function with a viewport bounds check and call it once at page load in addition to on scroll.

Will affect visual aspect of the product

No

Screenshots

Test instructions

• The charts should load when they come to viewport if lazy loading is enabled.
• Previously, the charts which were in viewport when the page loaded weren't loading until the user scrolls. Make sure that isn't the case anymore.

Check before Pull Request is ready:

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

Closes https://github.com/Codeinwp/visualizer-pro/issues/526.

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