Feature: add the if macro.

[tobymao]

No description provided.

[crericha]

Is this example still okay if we had:

"""select @if('a' = 'b', c), @if('a' = 'b', foo), d"""

which would be

select ,, d

[crericha]

sqlglot seems okay with it. Interesting feature that will be good for the docs.

Cleaner than having to put the commas in the if macro.