expose open-path for external use

While waiting for split views to land (#884), I wrote a [command](https://github.com/cldwalker/ltfiles/blob/509785f7faac1415c03398f6f70425989bf1122a/src/lt/plugins/ltfiles.cljs#L165-L181) which splits the current file. By splitting this out of the behavior, I'm hoping I can rely on LT's canonical implementation. I realize both open files don't keep in sync but for my use case, I only use the split view to keep one part of a file in view while editing another.