Change the data type of LogEntry.timestamp to `number`

[BALEHOK]

Let's change the data type here to number. I searched for all references in the code, and it's actually cast to string when set and then cast back to number everywhere in the app it's used 🤦

Originally posted by @BLoe in https://github.com/pixiebrix/pixiebrix-extension/pull/2705#discussion_r808520471