Best way to notify user about kernel.js change?

When an user has last installed the kernelspec ages ago, it will still work even if the kernel.js is outdated.

Is there an easy way to warn the user if the kernelspec directory is outdated?

Else I’d have to do something like

1. run `jupyter --data-dir`
2. find kernel.js in there and `stat` it
3. find out if the kernel package files are newer (in R: `find.package`, in python: `__file__`)
4. issue a warning if that’s the case

Start by examining the proposed `jupyter --data-dir` lookup, the `kernel.js` location, and the package paths exposed by R’s `find.package` and Python’s `__file__`. Define how an outdated kernelspec should be detected and when the user warning should appear; the issue does not name an implementation file or test.

Written by the indexing model from the issue text.