Update `.gitattributes`

[kubawerlos]

A lot of files is not export-ignored, and some files listed does not exists:

$ composer show | grep paraunit
facile-it/paraunit                                2.3.3  paraunit
$ ll ./vendor/facile-it/paraunit/
(...) ./
(...) ../
(...) .github/
(...) .php-cs-fixer.dist.php
(...) CHANGELOG-0.x.md
(...) CHANGELOG.md
(...) CONTRIBUTING.md
(...) LICENSE
(...) Makefile
(...) README.md
(...) codecov.yml
(...) composer.json
(...) docker-compose.override.yml.dist
(...) docker-compose.yml
(...) phpstan-baseline.neon
(...) psalm-baseline.xml
(...) psalm.xml
(...) rector.php
(...) src/

Resolves https://github.com/facile-it/paraunit/issues/242