Track AlterTableEvent once it is in a Datasette release

[simonw]

Problem! AlterTableEvent isn't in a release yet - I implemented that after I shipped 1.0a8.

Originally posted by @simonw in #53 (comment)

• Track alter table
• Track alter table when a table is renamed (set table to the new name)

This depends on:

• Alter table support for the JSON write API datasette#2101

[simonw]

My initial work on this for table renaming is here: 446c851