new_prop: Create new prop object

Description

The resulting object has the following fields:

new_prop(x, property, scale, offset, mult, env, event, label)

property The name of a visual property, like "x", "x2", "width", "y", "fill".
value A value. Can be a constant, reactive, or quoted expression.
scale A string with name of a scale. Typically something like "x", "y", "fill", but can also be a custom name like "foo".
offset Additive pixel offset used to adjust scaled values.
mult Multiplicative pixel offset used to adjust scaled values.
event A event like "update", "enter", "exit", "hover", "brush".
env An environment in which to evaluate a variable or reactive value.