Add a prop to HTML Axis components to use pixels and not percentages

[mhkeller]

Currently, the Axis components use percentages so they can be used in server-side charts. But you may want to use them also in normal charts that don't use percentRange if, for example, you want an easy to way to make a multi-line label in HTML.

All you have to do is change % to px and we could expose this a prop to make it easy for people.

[mhkeller]

This could also be done dynamically looking at the percentRange prop at first:

export let units = $percentRange === true ? '%' : 'px';