Access to old state on triggers

[xtrm0]

Describe the problem to be solved

It would be helpful to have a way to access row level old state in trigger formulas, similar to the way :old and :new work in SQL triggers https://www.ibm.com/docs/en/ias?topic=plsql-trigger-variables

Currently there is value, which reflect the old value on the target cell being updated, however, if the trigger is triggered by a change on some other column on the same row, there isn't any way to access the previous value of that cell without adding an extra hacky history column ( like history tracking example with extra column )

Adding row level :old, would make it much easier to add row level application logic to grist, without the need for external scripts / extra column hacks to manage history

See also discord thread

Related unanswered issues on the forum:
https://community.getgrist.com/t/row-history-timeline/1408
https://community.getgrist.com/t/webhooks-retrieve-the-old-value-of-a-clumns-when-un-updated-event-is-triggered/8112

Describe the solution you would like

Add a value row_old to grist function triggers - the current row before a trigger was triggered that contains the state of the current row before any updates happening on the row started.

For a given row, row_old should not change while trigger computations don't stabilize.