Migrate package enums to native PHP backed enums

[JonPurvis]

This PR replaces EnumTrait classes, from the funeralzone/valueobjects package, with native PHP backed enums that were introduced in PHP 8.1, now that the package requires that version as a minimum. We still require funeralzone/valueobjects for Value objects (ShopId, AccessToken, ShopDomain, etc.). There's multiple reasons we should do this:

• Less dependency surface
• Better tooling
• Clearer semantics
• Aligned with Laravel
• Future-proof

This change will most likely have to be a major version bump. If apps reference enums directly, then they will break (update instructions at the bottom of this comment)!

Changes

• All 11 enum classes are now native backed enums (string or int backing as appropriate)
• References updated throughout the package:
  • ChargeStatus::ACTIVE() is now ChargeStatus::ACTIVE
  • $enum->toNative() is now $enum->value
  • $a->isSame($b) is now $a === $b
• UPPERCASE case names preserved (e.g. ChargeStatus::ACTIVE, not Active) to minimise breaking changes. Whilst the general consensus is that enums should be CamelCase, I didn't think it was worth changing this.
• fromNative() kept but marked @deprecated on all enums. Folks should use Enum::from() / Enum::tryFrom() going forward. fromNative() could be removed in a later version.
• ChargeStatus::fromShopifyApiStatus() retained as a domain helper

Backing values

Enum ->value matches legacy toNative() output (constant name strings), not the old internal int constants.

Enum	Backing	Example ->value
ChargeStatus, PlanType, AuthMode, etc.	string	"ACTIVE", "RECURRING"
ThemeSupportLevel, SessionTokenSource	int	0, 1, 2

Breaking changes

Apps that reference package enums directly will need to update:

Before	After
ChargeStatus::ACTIVE()	ChargeStatus::ACTIVE
$status->toNative()	$status->value
$a->isSame($b)	$a === $b
ChargeStatus::fromNative('ACTIVE')	ChargeStatus::from('ACTIVE') (deprecated shim still works for now)