Support `CREATE TABLE x AS TABLE y`

[sarahyurick]

Hi, I'd like to be able to parse a statement like CREATE TABLE films2 AS TABLE films as from the second Postgres example found at this source. I currently tried to run this command with DataFusion but got a parser error, which is why I'm submitting the issue in this repo.

@andygrove I'm currently working on test_table_schema in dask-sql/test_schema.py which has a statement CREATE TABLE bar AS TABLE root.df, which is where the issue came from.