Improve the diffs app render queue

What does this MR do?

Improves how the rendering queue for the diffs app works by checking the idle time we have in the requestIdleCallback callback.

In this we also change the logic around renderIt to render the first 3 files quickly and then put the rest into the queue. This allows us to render quickly without blocking the browser a lot.

Screenshots (strongly suggested)

Before	After

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Closes #321691 (closed)