Fix Cashier migration order for new installations

[JaiveerChavda]

This PR fixes an issue where Cashier’s published migrations can run in the wrong order on fresh installations, causing child or alter migrations to execute before the base tables exist and resulting in migration failures.

On a new project, after publishing Cashier migrations, their filenames may be sorted alphabetically in a way that breaks the intended execution order. This behavior is documented in the following issue:

• Migrations fail for new installs #1810

To ensure deterministic and correct ordering, this change prefixes the published migration filenames with explicit sequence markers (_1, _2, _3, …). This guarantees that:

• Base tables are created before dependent or altering migrations.
• Fresh installs migrate cleanly.
• Existing projects are not affected, as their core tables already exist.

This aligns Cashier’s migration behavior with developer expectations and prevents new users from encountering setup failures.

[taylorotwell]

Nothing fails for me on a fresh Laravel project, installing Cashier, publishing migrations, and then running them.