Improve enforcement of tok constraint

[amir-zeldes]

Currently spreadsheet mode assumes we have a tok column, by convention in spreadsheet column A. If this column does not exist, bugs crop up in various places, for example a key error in validation here:

https://github.com/gucorpling/gitdox/blob/dev/modules/validate_spreadsheet.py#L215

It might be better to either really assume that column A is tok (even if unlabeled), or to do something like this:

• Look for a column named tok
• If it's not found, throw a validation error but try assuming column A is tok instead of crashing
• Regardless of the above operations, check that the tok column or column A if not found have no spans - if they have a span, report this and fail the validation check without crashing