Added php8.0 features to Resizer

[jaxwilko]

A simple PR that adds some php8.0 features and enforces return types.

Something of note, all the widths & heights should probably be ints but in reality they work off floats and changing them to int breaks the tests.

This PR lays groundwork for future improvements.

[jaxwilko]

There maybe a bug when trying to resize in crop mode where the calculated values (float) for width & height are passed to crop() and implicitly cast to int resulting in a potential issue.

Need to add more tests for better coverage of the $resizer->setOptions(['mode' => ...]) feature.

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[LukeTowers]

@jaxwilko can you remerge develop into this branch and then let me know what's left on it?