Added functions for finding and deleting syntax node descendants.

Added syntax_node_alloc_descendants_with_type() and
syntax_node_delete_descendants_with_type() functions.

Renamed syntax_node_span() to syntax_node_text_size().

Changed implementations of template_parser_trim_parse_tree() and
sitedef_parser_trim_parse_tree() to call
syntax_node_delete_descendants_with_type().