[css-forms-1] The "slider" controls might want composability

[mfreed7]

For example, a developer might want to provide the thumb (or each thumb?) of an <input type=range> as a custom element widget. For example, a thumb that can display the current value of the slider in some cool way.

Or developers might want to include custom content on the track, or for the <datalist> tickmarks and labels, or other things.

Feels like there should be a way to provide arbitrary HTML (like customizable-<select>) for this control.

[lukewarlow]

Somewhat related #11873