**Use case**

I want to estimate better parameters for fulltext (`ngrambf_v1`) index.

**Describe the solution you'd like**

Implement it as SQL function:

`ngrams(string, N)`

that returns array of FixedString.

**Describe alternatives you've considered**

It can be done with less efficient and more cumbersome way:

```
WITH 3 AS n, extractAll(text, '.') AS chars 
SELECT 
  arrayMap(x -> arrayStringConcat(x), 
    arrayFilter(x -> length(x) = n, 
      arrayMap(x, i -> arraySlice(chars, i, n), 
        chars, arrayEnumerate(chars)))) AS ngrams
```

**Additional context**

`ngramsUTF8` can also be implemented to extract ngrams of Unicode code points.
{
            
            FUNCTION_EQUALS,
            FUNCTION_NOT_EQUALS,
            FUNCTION_HAS,
            FUNCTION_IN,
            FUNCTION_NOT_IN,
            FUNCTION_MULTI_SEARCH,
            FUNCTION_UNKNOWN, 
            
            FUNCTION_NOT,
            FUNCTION_AND,
            FUNCTION_OR,
            
            ALWAYS_FALSE,
            ALWAYS_TRUE,
        };
        RPNElement(