parsePathString does not fully implement arc BNF

[ish-kafel]

According to elliptical-arc-argument BNF in this section

elliptical-arc-argument:

nonnegative-number comma-wsp? nonnegative-number comma-wsp?

number comma-wsp flag comma-wsp? flag comma-wsp? coordinate-pair

there could be nothing between two flags, but parsePathString does not support. Could be possible to implement this syntax?

[nikku]

Happy to take a PR that implements this feature request (and a valid test case).