Remove transaction export option?

[JAForbes]

The more migrations I do the more I find I don't use the transaction option because I frequently am using SQL statements that cannot be run in a transaction.

And the drop owned by auto teardown works so well in development that atomicity isn't really needed.

So I'm thinking instead of dropping support for export const transaction and only supporting export const action and as there's only one type of operation at that point, action can just be export default as well.

If a user wants a transaction they can just use sql.begin, which we can document and then refer to postgres.js docs