Store "transactions" to enable undo/redo history

Kind of based off of the data structure in https://github.com/validated-changeset/validated-changeset/issues/144

It could be good to record "transactions" every time execute is called, effectively storing that current/original object from #144 as a type-of-snapshot each time execute is called. Or maybe it'd look like:

```js
changeset.transactions
// returns =>
[
{
executedAt: timestamp,
snapshot: (existing snapshot)
changes: {
key: { original, current }
}
},
{
executedAt: timestamp,
snapshot: (existing snapshot)
changes: {
key: { original, current }
}
}
// ....
]

```

This would enable undo/redo history