Refactor Source to use fs.FS under the hood.

[SpeedyCoder]

This will allow users to pass in any FS implementation. For example embed.FS, that allows usage of embedded migration files. Given that fs.FS is read only I had to modify the Create functionality. I've removed the method from the Source struct and moved the logic into the Create function and changed its signature to take path instead of Source struct. This is only a helper function, so I think even though it's a breaking change it should be fine to make.