Add Internal loading Prop for Large Diffs

[bpetii]

Problem Statement
When handling large diffs, react-diff-viewer-continued can cause some performance issues, leading to UI freezing while parsing large inputs.

Proposed Solution
Introduce an internal loading prop that allows developers to customize the loading state when rendering large diffs. This could be implemented as:

A customizable loadingText prop to render a custom loader.

[reinrl]

+1 (just came back to the docs to see if this was already a thing, and then headed to issues next)

[yangzi0210]

+1 @Aeolun
This is very important. When it exceeds 1k lines, there will be some lag. Using a loading state can easily but effectively solve this problem.
If you could help, I would greatly appreciate it！

[puneet-goel]

Hi @Aeolun ,

I'd love to help with this! Could you please assign this issue to me?

I can start working on implementing the loadingText prop, and I also have another approach in mind — using a virtualized window to render only the content currently visible in the user's viewport. This would significantly optimize the rendering performance.

Thanks a lot!