Show code of the source on click of the source box at the top of the observable

[souvikbasu]

The observable has 3 parts visually, the top source, the middle stream and the bottom destination.

As part of this issue, if the user clicks on the top source box, then they should be able to view the code behind that generates the input for the stream.
For example the source may be a timer that generates numbers every second. So on click of the source box, the user should be able to see the code in a panel as:

const source = timer(1000);