migration/definition: accept named migrations

Created by: bobheadxi

Implements the first step of https://github.com/sourcegraph/sourcegraph/issues/37244:

modify readDefinitions in internal/database/migration/definition/read.go to ignore anything after the first _ in the Atoi call

Test plan

Unit tests show this works for both existing migrations and migrations that have names in them

go run ./dev sg migration leaves <commit> works