Add support for dropping multiple columns in Snowflake

[yoavcloud]

This is not documented but works. For example:

create or replace table multi_drop (id1 int, id2 int, id3 int);
alter table multi_drop drop column if exists id1, id2;
desc table multi_drop; -- shows id3

[iffyio]

LGTM! Thanks @yoavcloud!
cc @alamb