feat: include concat operator

[Michad]

Currently you get an error if you use SQL with concatenation. As far as I'm aware, the || operator is part of ANSI SQL and it seems simple to support.

I noticed there's a comment

/**
 * Borrowed from PL/SQL ,the priority of below list IS ORDER BY DESC
 * ---------------------------------------------------------------------------------------------------
 * | +, -                                                     | identity, negation                   |
 * | *, /                                                     | multiplication, division             |
 * | +, -                                                     | addition, subtraction, concatenation |
 * | =, <, >, <=, >=, <>, !=, IS, LIKE, BETWEEN, IN           | comparion                            |
 * | !, NOT                                                   | logical negation                     |
 * | AND                                                      | conjunction                          |
 * | OR                                                       | inclusion                            |
 * ---------------------------------------------------------------------------------------------------
 */

Which seems to suggest concatenation should be grouped in with addition and subtraction, so that's what I did.

[mbaumgartl]

Thanks for the pull request! Unfortunately, it currently doesn’t handle operator precedence correctly and it also lacks tests. In addition, implementing the operator properly would add complexity to the codebase for what is essentially syntactic sugar.