NodeFilter was defined twice: a partial live definition (SHOW_ELEMENT,
SHOW_TEXT, SHOW_ALL) and a complete one behind an `if (typeof NodeFilter
=== 'undefined')` guard that never fired because the partial one already
defined it. So NodeFilter.FILTER_ACCEPT/REJECT/SKIP and most SHOW_* values
were undefined at runtime, and the canonical filter idiom
`acceptNode() { return NodeFilter.FILTER_ACCEPT; }` returned undefined —
making TreeWalker/NodeIterator reject every node.
Make the live definition authoritative with the full standard constant set
and drop the dead duplicate. Adds tests asserting the constant values and
that a filter returning NodeFilter.FILTER_ACCEPT accepts the node.
Andrey Mnatsakanov
c3ae054dde