dia.attributes: support negative number addition/subtraction in calc(…

[kumilingus]

…) expression

Support calc() expression in these forms:

calc(multipy * property - -number)
calc(multiply * propery + -number)

This is common when values are inserted into the calc() expression via template literals.

const offset = -20;
const d = `M 0 0 calc(w-${offset}) 0`;
const d = `M 0 0 calc(w+${offset}) 0`;