clean up php 8.4 and other changes (#156)

use phpunit depends notation to order tests

remove duplicate file

Add optional ConfirmationPersonalNote to createSenderSignature (#131)

Issue 129 (#130)

This addresses issue #129.

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Co-authored-by: Ash <1744544+ashgibson@users.noreply.github.com>
Co-authored-by: Eli Wood <ewood@PC26VM7WKL.local>

Fix fatal error when headers is null (#128)

Headers defaults to "null" if empty, and "fixHeaders" can return null
too. Then we should allow "null" for the setHeaders function.

Exception2 (#127)

An attempt to fix some issues that are not handled gracefully.

It seems as though on occasion there is an issue within the exception
handling. This tries to solve that.

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Co-authored-by: Ash <1744544+ashgibson@users.noreply.github.com>
Co-authored-by: Eli Wood <ewood@PC26VM7WKL.local>

Fixed getSuppressions return type (#126)

Fixed getSuppressions to return PostmarkSuppressionList instead of
PostmarkSuppressionResultList

Split classes in their own file (#124)

Some classes were stored in the same file, not following PSR4
conventions.

This prevented tools such as PHPStan to easily discover these symbols &
read these classes.

Moving them to different files does not cause a breaking change as the
namespace does not change.

Fixes for some issues (#123)

- Address [issue
121](#121)
- Address [issue
122](#122)

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Co-authored-by: Eli Wood <ewood@PC26VM7WKL.local>