Add option to toggle trailing commas in SELECT columns list

[Rene Saarsoo]

Currently the plugin strips trailing commas to enforce a consistent syntax across all SQL dialects. Trailing commas are not part of the SQL standard and are only supported by a few dialects.

However, in dialects that support them (like BigQuery), it's a really nice feature. In other dialects many users resort to using comma-first style to solve the problem for which trailing commas is a much nicer solution.

Something like sqlSelectTrailingComma: "always" | "never" | "preserve". Possibly the "preserve" isn't even needed as it opens a door for inconsistent style.