Support CSSNumericValues as inset

[johannesodland]

Depends on https://github.com/flackr/scroll-timeline/pull/176

This PR adds support for passing CSSNumericValues as insets to ViewTimeline.

new ViewTimeline({
  subject: elm,
  axis: 'block',
  inset: [ CSS.percent(10), CSS.percent(20) ]
})

To pass the tests the PR also do the following:

• Adds MutationObserver to observe changes to class attribute on subject.
• Adds getters for startOffset and endOffset read only attributes.
• Adds support for parsing math functions such as calc, through a naive implementation of CSSNumericValue.parse()

This PR does not:

• add support for relative units such as viewport relative units.

[flackr]

Uhoh, looks like my naive diff of test results approach has encountered it's first issue with what seems to be a flaky test.