start and skip attributes must be namespaced

The skip and start attributes are not namespace, which can cause issues (see comments on NBREF-12 and NCAPP-15)

Rename skip to data-numbered-headings-skip.
Rename start to data-numbered-headings-start.

Prefixed with data to follow https://developer.mozilla.org/en-US/docs/Learn/HTML/Howto/Use_data_attributes
Use numbered-headings for the Numbered Headings application namespace
The last segment is the attribute name.