Support standard 'locate' SQL function

[dfulachier]

ClickHouse proposes the 'position' function, and we can do with that.
But in some case (queries generated from a BI tool such as Tableau), we cannot master the SQL generated. So it may be fine to get the locate function, with both possible syntaxes:

• locate(search, source)
• locate(search, source, startPos)

[blinkov]

Alias added in #3774, but position don't support startPos optional arg at the moment.