Support linear regression transform

[jonmmease]

DataFusion recently got support for linear regression aggregate functions (apache/datafusion#7211). Postgres and DuckDB also support these functions (I haven't looked into other dialects).

We can use these to implement the vega regression transform for type == "linear".

With a bit of manipulation, we should also be able to implement log(x) regression. I don't think the other regression types are expressible in terms of linear regression though.