Save scroll animation status in visit object

[daun]

Description

• Add a visit.scroll.animate key and use it to determine whether to animate scrolling or not
• Allow disabling scroll animations via the visit object
• Will be used in an upcoming prefers-reduced-motion feature of the Accessibility Plugin

Drive-by improvement

• Fix same scroll reset issue fixed in core recently
• Make second animate argument of swup.scrollTo() optional
• Rename scroll handler for consistency

How to disable scroll animations

// Disable scroll animation between pages
swup.hooks.before('visit:start', (visit) => visit.scroll.animate  = false)

// Disable scroll animation on link to same page
swup.hooks.before('link:self', (visit) => visit.scroll.animate  = false)

// Disable scroll animation on link to same page anchor
swup.hooks.before('link:anchor', (visit) => visit.scroll.animate  = false)

Checks

• [x] The PR is submitted to the master branch
• [x] The code was linted before pushing (npm run lint)

Additional information

Closes #63