Code style updates, Receipt error updates

Merge pull request #15 from DeBoerTool/error

Convert error codes to an enum

Make Receipt JsonSerializable

• Also adds a ReceiptTest

Add missing null return type

Merge pull request #10 from DeBoerTool/fix-return-type

[fix] Include null in return type

ErrorList and ErrorInterface implent JsonSerializable

Merge pull request #9 from DeBoerTool/error-json

Error json

Initialize errors on Response

Merge pull request #7 from DeBoerTool/fix-response-errors

Fix response errors

Update constructor signatures, add Enviroment, update validation

• See changelog for more details.

Add interfaces for Moneris and Gateway

• For applications with a container, you can hang bindings on those interfaces.

Add GitHub Action to run PHPUnit checks on PR

• Additionally updated some badges and whatnot

Upgrade to PHP 8

• Change Vault::update(...) signature to prevent PHP 8 from throwing an parameter order error
• Require PHP 8
• Require newer versions of PHPUnit, Mockery, and Faker
• Update test setUp signature to match PHPUnit
• Update phpunit.dist.xml to new schema