add halfOfYear, isFirstHalfOfYear, IsSecondHalfOfYear; improve getter…

[brenoroosevelt]

I would like to propose the inclusion of the halfOfYear property and the isFirstHalfOfYear and isSecondHalfOfYear methods. The first half of the year corresponds to the first 6 months, while the second half corresponds to the remaining months.

Furthermore, I've made some improvements to the type hinting of the read-only properties in order to specify the numeric range. For example, changing daysInMonth from int to int<1, 31>. This allows for better static code analysis by PHPStan and Psalm.

Thank you in advance.

[brenoroosevelt]

@othercorey Changes made. renamed to 'half'

[othercorey]

Thanks!