MDEV-10267 Add "ngram" support to MariaDB

[i-rinat]

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Ngram is a fulltext parser plugin that splits words into overlapping segments of fixed lengths. For example, if 3-grams are in use, string "abcdef" is split into "abc", "bcd", "cde", "def", thus allowing an efficient substring search for CJK texts where splitting by words in not feasible. Only word characters are going into n-grams. Spaces and punctuation characters are treated as separators.