Support CJK index creation and query

[xiaoyifang]

Current situation:
https://download.kiwix.org/zim/wikipedia/wikipedia_zh_mathematics_mini_2023-05.zim

fulltext search 平方
gives no result.

actually there are much more articles contain this word.

english word seems ok.

The No seems another issue.

CJK characters have to be indexed with CJK flags

Creation:

   Xapian::TermGenerator indexer;

    indexer.set_flags( Xapian::TermGenerator::FLAG_CJK_NGRAM );

Query:

      Xapian::QueryParser::feature_flag flag = Xapian::QueryParser::FLAG_DEFAULT;
      if( searchMode == FTS::Wildcards )
        flag = Xapian::QueryParser::FLAG_WILDCARD;
      Xapian::Query query = qp.parse_query( query_string, flag|Xapian::QueryParser::FLAG_CJK_NGRAM );

Originally posted by @kelson42 in #794 (comment)

[mgautierfr]

@xiaoyifang The PR #806 should fix this.
Can you test on your side ? I'm not sure to fully understand what is the expected behavior :)

[xiaoyifang]

From the unittest you provided .it seems ok.