Add type hint, model hint, and timestamp cast

[rxrw]

1. Update phpunit.xml to new version
2. Add type hint to all functions
3. Add properties hint in all models
4. (BREAKING CHANGES)Add casts to Carbon for timestamp field

[rxrw]

Hi, I have found this project which suits my requirement! I'm very appreciating your work.

In the process of development, I found there're some difficulties getting hints from IDE.

Above PHP 8 and Laravel 9, there's a recommendation to use type hint in order to make code more readable and easily to develop.

I have done them. Maybe the method hint in model is not fully supported, but it's enough to use. Many contributors and me will continue to make it more completely.

Please let me know if there're any issues, and I'm looking forwards to this PR to be merged ^-^

Due to the breaking changes in timestamp type, I think maybe there should have a major version change 🤔

[lucasdotvin]

Hello! Thanks for the contributions! I'm gonna run some tests and then merge it! 🤘

[codecov[bot]]

Codecov Report

Merging #69 (1d1f52a) into develop (86d227c) will decrease coverage by 1.01%. The diff coverage is 99.07%.

@@              Coverage Diff              @@
##             develop      #69      +/-   ##
=============================================
- Coverage      98.20%   97.19%   -1.01%     
  Complexity       150      150              
=============================================
  Files             26       26              
  Lines            501      499       -2     
=============================================
- Hits             492      485       -7     
- Misses             9       14       +5     

Impacted Files	Coverage Δ
src/Enums/PeriodicityType.php	100.00% <ø> (ø)
src/Models/Concerns/HasSubscriptions.php	96.00% <96.77%> (-0.02%)	:arrow_down:
src/Models/Concerns/Expires.php	88.88% <100.00%> (-11.12%)	:arrow_down:
src/Models/Concerns/ExpiresAndHasGraceDays.php	88.88% <100.00%> (-11.12%)	:arrow_down:
src/Models/Concerns/HandlesRecurrence.php	100.00% <100.00%> (ø)
src/Models/Concerns/Starts.php	90.90% <100.00%> (-9.10%)	:arrow_down:
src/Models/Concerns/Suppresses.php	90.90% <100.00%> (-9.10%)	:arrow_down:
src/Models/Feature.php	100.00% <100.00%> (ø)
src/Models/FeatureConsumption.php	100.00% <100.00%> (ø)
src/Models/FeaturePlan.php	100.00% <100.00%> (ø)
... and 9 more