Syntax: migrate models to leading commas

[rlh1994]

Currently most (although not all) models use trailing commas, is it more common for basic models and has so far worked with no issue. As we add more optional field, such as those added by the conversions macro, we sometimes have to use leading commas as these fields are the last in a model but may or may not exist and not all warehouse support a hanging last comma.

This means we either need to make the comma itself on the model conditional on the variable and just trailing in the macro (ugly), mix existing trailing with leading in the macro (inconsistent), or move the whole model to leading (PR touching basically every line).

At some point, if/when we do introduce sqlfluff this is likely to be one of the core rules added first, so it makes sense to discuss and agree now.

Most cases I have encountered gain additional flexibility from using leading, as much as I personally prefer to use trailing, but there may be cases for trailing I haven't considered.

[georgewoodhead]

My preference has always been trailing but I think leading commas makes more sense now and we should be consistent everywhere. I do think it’s important we have sqlfluff in place if we decide to make this switch as it is going to be hard to break trailing comma habits! It would also make the change over work easier.

[rlh1994]

Yeah that makes sense, I think we should wait until the package development has got quieter so we can make that change via a PR and not touch anything else at the same time!