feat(kernel): provide warnings about kernel changes

[brettc]

Summary

Currently, a kernel (python / bash / r) is located by just finding the first available hit on the path. This might change, depending on what paths are active in different environments on the same computer. When it does change, this can impact what happens with assistants (as different packages / versions may be installed). Ideally, we would lock stencila to particular environments, but this will be too much for many users. However, we could warn them about changes since it was last run.