Add convenience method for parsing simple SELECT

It would be nice to have a `parse_select` method that returns `SQLSelect` struct rather than have to deal with `SQLQuery` and `SQLSetExpr` for simple use cases.

Start by locating the existing parsing entry points and the definitions of SQLSelect, SQLQuery, and SQLSetExpr. Confirm how simple SELECT statements are currently represented, then add coverage showing that parse_select returns SQLSelect for those cases; done means the convenience API works without callers handling SQLQuery and SQLSetExpr.

Written by the indexing model from the issue text.