feat(migrations): implement create command

[vcrfxia]

Description

The migrations create command creates an empty migrations file, of the appropriate filename format.

Testing done

Unit + integration.

Reviewer checklist

• Ensure docs are updated if necessary. (eg. if a user visible feature is being added or changed).
• Ensure relevant issues are linked (description should include text like "Fixes #")

[jzaralim]

We should also do this check in apply, since validate only checks for duplicate versions for versions that have already been migrated. Currently, apply will still run if there are two unmigrated files with the same version number. This can be done in a follow-up.

[vcrfxia]

Sure, I'm working on a follow-up with some changes to apply right now anyway, so I'll slip this into my next PR. Good call!