feat: update library autoloading to psr-4

[adevade]

Description

Move from PSR-0 to PSR-4 autoloading

This PR changes autoloading from using PSR-0 to PSR-4. PSR-4 is used by basically all modern PHP projects. Main difference is project file/directory structure.
All source and test files have been moved. Namespace has been added to the tests as well.

PSR-0 has been deprecated since 2014, and PSR-4 is recommended as replacement.

Read more on Stack Overflow

I've also removed the custom autoloader found in src/, since Composer always includes one. If not using Composer you can always manually include/require the files since they're so few.

Add config file for PHPUnit

Added phpunit.xml config file to repo root, so vendor/bin/phpunit can be called without duplicating arguments everywhere.

Checklist

• Read the contributing guidelines.
• Each commit follows the Conventional Commit spec format.
• Update the readme.
• All existing unit tests are still passing.

[commit-lint]

Config

• switch to psr-4 autoloading (f51b1d8)
• remove custom legacy autoloader (35fa2ef)
• add config file for phpunit (c0d7932)
• gitignore phpunit cache file (79de5de)
• remove custom legacy autoloader (0c9b3d6)

Code Refactoring

• move src files for psr-4 (5cd223b)
• move test files and add namespace (fde5924)

Documentation

• remove reference to old autoloader (de16dd8)

Styles

• remove trailing whitespace (3271b33)

Contributors

adevade

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[sherwinski]

LGTM
Only comment I have is that a lot of the spacing clean up here seems duplicated from #96.
I'm going to rebase next onto main (now that the aforementioned PR is merged) so that should hopefully reduce any replications in this PR.

[luqven]

Resolved merge conflicts, waiting on CI to do its thing.