apply the appropriate language configuration as the selected cell changes

[brettfo]

This allows the comment/uncomment keyboard shortcuts Ctrl+K,C/Ctrl+K,U to work based on the language of the current cell. This also allows things like auto-complete to fill in ) after a ( is typed, etc.

This doesn't work, however, if the cursor is after a kernel selector magic command like this:

// this is a C# cell

#!python
print("hello from python")

If the user tries to comment the print(..) line it'll be replaced by // print(...) instead of # print(...) because we have no way of detecting the cursor's location, and thus the current language at the cursor. If we did it would be trivial to see that we were in a #!python selector and to the right thing, but since polyglot cells aren't as common, this is a shortcoming we'll have to deal with.

Fixes #2559.

[AbhitejJohn]

we have no way of detecting the cursor's location, and thus the current language at the cursor

Would this be in the VS Code layer? Perhaps we could file a feature request, so this can be made simpler in the long term.

[brettfo]

@AbhitejJohn Slight correction to my statement above: we have no way of detecting when the cursor location changes, so we can't respond to the user arrowing around in the file. I'll think about exactly what we would need and submit a suggestion.