Show placeholder for R kernel and Python kernel upon opening polyglot notebook

[sharoncxu]

Is your feature request related to a problem? Please describe.
Users currently have no way of knowing there is python or R support through UI

Describe the solution you'd like
Show a placeholder spot for R and Python in the language picker (just like how SQL is treated) and delay connection to the kernel until the user runs the connection string. If the user attempts to run code using Python or R before completing the connection, display the instructions to connect to a Python/R kernel

[jonsequitur]

Consider using Python environment tools for more efficient environment discovery.

[jonsequitur]

Currently, the closest thing to this is under the Connect to new cell kernel command:

This will insert cells into the notebook rather than simply running the package acquisition (if needed) and #!connect command. This makes the resulting notebook runnable on Jupyter frontends that don't have the supporting custom UI.