Type-specific popup editors for certain expressions

Certain expressions in code have types that are easier to edit in other ways than text. When we find a literal expression of these types, such as `RGB(0.5, 0.4, 0.2)`, or `polygon[(1, 2), (4, 2), (3, 3)]` containing no variables, we could offer a button on hover that opens that expression in a type-specific editor, such as an RGB color chooser, or a drawing interface for drawing polygons on a coordinate plane.

Some other type info can be inferred from the containing context, so for example a number could be interpreted as an angle because it's used as an argument to `rotated`.