convertShapeToPath: Convert rectangles with radius to paths

[santhoshtr]

Currently rect shapes with rx or ry radius attributes are not converted to path by convertShapeToPath plugin.

This PR adds support for converting those radius to paths under convertArcs setting of convertShapeToPath plugin.

Reference spec: https://www.w3.org/TR/SVG11/shapes.html#RectElementRXAttribute

Usecase example: When icons are converted to single path using mergePaths with force=true setting icons with at least one rectangle with given radius is left out. This is problematic for single path icons patterns followed by materialicons, or fontawesome and others.

[moatorres]

I guess it should be node.attributes.width instead of item as per 2.7.0

[moatorres]

Not sure if you've forked the current version, but this code seems to work if we change item to node as in:

enter: (node, parentNode) => { ...

But using let rx = Number( ... actually messes up the code as this variable name is used down the line.

    // optionally covert ellipse
        if (node.name === 'ellipse' && convertArcs) {
          const ecx = Number(node.attributes.cx || '0');
          const ecy = Number(node.attributes.cy || '0');
          const rx = Number(node.attributes.rx || '0');
          const ry = Number(node.attributes.ry || '0');
          if (Number.isNaN(ecx - ecy + rx - ry)) {
            return;
          }

Maybe it could be named rry to avoid conflicts? Shooting in the dark here.