Laravel 9+ features

[judgej]

Some nice-to-haves, not all essential.

• Support PHP Money ^4.0
• Use PHP enum for ledger type.
• Use string for accounting_ledgers.type. Database enums are great, but really hard to manage in Laravel.
• Use anonymous function style migrations.
• Database migration should use the current date and time (but keep the migrations in the correct order).
• Add notes about using aliases for the morphed type models.
• Replace dates casting with casts in the models (dates has been deprecated, and removed for Laravel 10).
• Fix balance calculations so they cannot accidentally mix currencies.
• Support Carbon\CarbonImmutable for dates, as that is the new default for Laravel 9+. Use Carbon\CarbonInterface as the common interface.
• initJournal should take a Money\Currency object to be sure it is valid, and if not supplied, default to the config currency, not hard-coded USD.
• Fix model needing to be re-fetched after its journal has been initialised before the journal can be seen.
• The transaction reference object should be a normal polynomial relationship.
• Unique index on journals to ensure a model does not have multiple journals.
• Move the migrations out of the autoload src directory.
• Overhaul the whole readme.