New DataStore.fileExists(pathname) API call

Now that we're using shared libraries, we could do with a way to check files are present. Currently we have to use .loadData(pathname) to check for this. Some of these files (react and fonts) are quite large, and we're concerned about the time it takes to load them just to test if they are there and throw the loaded file away. Better to have a DataStore.fileExists(pathname) call, which would be nearly instantaneous.

[edit to add] More importantly, I now realise, it avoids an error being thrown, which currently is what happens if pathname doesn't exist.