Add gutter option

[lozcalver]

This was simpler than I expected! I haven’t added a test - I’ve never used vitest, so wasn’t really sure where to start.

There’s one notable difference between this and the webpack plugin’s version - I haven’t included the following calculations:

https://github.com/cascornelissen/svg-spritemap-webpack-plugin/blob/775c4b63cb7e2287201d2331544ed677a549ee84/lib/generate-svg.js#L302

As far as I can tell, these calculations adjust the viewBox size on the <view /> elements to add 1/2 the gutter size around the sprite. However, this just results in extra space appearing around the sprite when the -view is used, so I’m not sure why they added this. In my testing it works much better without that additional spacing.

The commit that introduced those calculations is this one: cascornelissen/svg-spritemap-webpack-plugin@6612187. I think it may be a bug that nobody has picked up on because <view /> is used less frequently than <use />.