I am still learning Luxor.
I find two in-built tangent functions: circletangent2circles() makes circles of a particular radius tangential to two circles circlepointtangent() makes circles of a particular radius passing through a point and tangential to another circle

But I do not find in-built functions for other more common tangents,
like a tangent from a point to a circle, or a common tangent of two circles.
Is there any plan to add these functions?

But you could raise issues like this over at the github repository, where open issues provide a visible reminder of features that could be added should anyone have the skills and inclination to do so.