Add asyncInterval.js to MoDiBo

This is a simple one. Add `asyncInterval.js` from [Spike-2](https://github.com/spike-2/Spike-2/blob/main/src/asyncInterval.js) to `/src` so future code can use it. This should literally be a copy-paste operation.

Compare src/asyncInterval.js in Spike-2 with the MoDiBo repository's /src directory. Copy the referenced file into /src, then verify that the file matches the upstream version and is available for future code to use.

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