Add option (or default) to remove `xml:space` as it is deprecated

[perpetual-education-personal]

https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/xml:space

<svg viewBox="0 0 10 10" xml:space="preserve" ...

[XhmikosR]

Upstream PR svg/svgo#1660

[XhmikosR]

Closing for now. If it's implemented upstream, we can update svgo and add the plugin here too.