Color code columns that contain a formula

Both on the table header itself (like a grayed out background) and lists of variables when selecting a column name while editing a formula: it would go a long way in visually facilitating building formulas (it is helpful to see one is “chaining” multiple formulas).

At some point, it would be nice to visualize all dependencies in a formula (like Excel has a trace function for formulas - I can’t overstate how immensely helpful that is for debugging them!)