Redesign WP_HTML_Template API for table context support

Replace the eager from()->bind()->render() chain with two static
entry points: render() and template(). Templates are now processed
in a single pass, and nested templates are parsed at render time in
the parent's context via create_fragment_at_current_node(), enabling
correct handling of table elements (<tr>, <td>, etc.) that require
non-body insertion modes.

Author: sirreal