Show expression values in the expression result area

[jared-hughes]

In vanilla Desmos, various expressions' results are shown in a preview in the expressions list (https://www.desmos.com/calculator/ufiwm7unbp).

This could be useful to implement for some DesThree types. Example path:

• Position the camera using spherical coordinates as such: PerspectiveCamera((Rcos(a)cos(b), Rsin(b), Rsin(a)cos(b))
• Wonder: what are the Cartesian coordinates of the camera?
• Copy-paste the x, y, and z expressions into separate Desmos expressions
• Graph is no longer DRY, and this is more work

It would be helpful to see the three coordinates in the expressions list.