add support for non-timestamp but still numeric migrations

[psteinroe]

Discussed in #298

^{Originally posted by mplanchard April 3, 2025}
Right now, it seems like the after configuration for migrations is closely coupled with a file format used by certain ORMs (e.g. prisma).

In our project, the files are named like 0001_whatever.sql, with no timestamp.

If after is going to refer to a timestamp rather than a migration number, it would be nice to do it in a more generalized way. The first thing that comes to mind is using git to determine the file's last modification time, if VCS integration is enabled.

Alternatively, a configuration option to use migration numbers would be nice, which could potentially take some kind of DSL to convert filenames to migration numbers (perhaps a sed command).