BUGFIX-105: fix radar chart dragging to be more precise

[YBCoding]

Hello @chrispahm, here a PR to change the dragging of radar chart. I'm not an expert on this so if you have any suggestion =) I think it solves mostly the problem of dragging things to 0.

Description :

• Use getRelativePosition, getValueForDistanceFromCenter, getPointPositionForValue as suggested in https://github.com/chartjs/Chart.js/issues/5931

• distance calculation was using pythagoere theorem making the dragging a bit weird near 0 and in opposite side of the axis (due to the square root). we changed it so that it uses consinus, and at x = 0 it uses the cursor Y position relative to the center

Closes #105

[YBCoding]

@chrispahm small bump, do you need any help to integrate this PR ?