feat(convertShapeToPath): Optionally convert rects with rounded corners to paths

[baarde]

Currently, rectangles with rounded corners are excluded from the convertShapeToPath optimization.

This PR adds the conversion of rectangles with rounded corners to paths when the convertArcs option is enabled.

Though the converted <path> is quite longer than the source <rect>, the conversion is still useful to people who wish to merge all the shapes into a single path.

[baarde]

Special cases

• rx is absent → ry is used instead.
• ry is absent → rx is used instead.
• rx or ry is equal to zero → The <rect> is converted without rounded corners.
• rx or ry is negative or not a number → The <rect> is not converted.
• The computed value of rx is greater than half the width → rx is clamped to half the width.
• The computed value of ry is greater than half the height → ry is clamped to half the height.

[baarde]

I've just found out there's already a similar PR (#1816), which I totally missed. Sorry for the dupe. 😅