feat(sql): add as method to SQL.Aliased

[aleclarson]

Add SQL.Aliased#as(alias) method, allowing you to alias a subquery column.

When using db.$with().as(), a subquery is returned, whose columns may be a SQL.Aliased instance. If you want to reference a subquery's column in another subquery, but use a different column name, this PR will make it easier to do that.

Current workaround

The drawback of this workaround is it doesn't automatically inherit the type from the referenced column, so you have to manually specify it, which is fragile.

sql<string>`${subquery.foo}`.as('alias')