chore: stricter PHPStan code

.gitlab-ci.yml

.php-cs-fixer.php

Makefile

README.md

composer.json

composer.lock

phpstan.neon

phpunit.xml

src/Base64/Base64Encoder.php

src/Base64/Base64UrlSafeEncoder.php

src/Book/ArrayBook.php

src/Book/BookEncoder.php

src/Book/OneByteBookEncoder.php

src/Book/TwoBytesBookEncoder.php

tests/Base64/Base64EncoderTest.php

tests/Base64/Base64UrlSafeEncoderTest.php

tests/Basic/HexadecimalEncoderTest.php

tests/Book/BookEncoderTest.php

tests/Book/OneByteBookEncoderTest.php

tests/Book/TwoBytesBookEncoderTest.php