[feature] implement CASE expression

[bohutang]

Summary

Syntax:

CASE <cond>
  WHEN <condval1> THEN <expr1>
  [ WHEN <condvalx> THEN <exprx> ] ...
  [ ELSE <expr2> ]
END

For example:

SELECT CustomerName, City, Country
FROM Customers
ORDER BY
(CASE
    WHEN City IS NULL THEN Country
    ELSE City
END);