Setup .gitattributes - in order to exclude tests and other dev files from dist

[sasezaki]

• Currently, The archive targets other than src/ are as follow

gh api repos/phpDocumentor/GraphViz/tarball/HEAD | tar -tzf - | cut -d/ -f2- | sed '/^$/d' | grep -v src/
.github/
.github/dependabot.yml
.github/workflows/
.github/workflows/push.yml
.gitignore
.scrutinizer.yml
LICENSE
Makefile
README.md
composer.json
phpcs.xml.dist
phpmd.xml.dist
phpstan.neon
phpunit.xml.dist
psalm.xml
tests/
tests/Resources/
tests/Resources/Dockerfile
tests/phpDocumentor/
tests/phpDocumentor/GraphViz/
tests/phpDocumentor/GraphViz/Test/
tests/phpDocumentor/GraphViz/Test/AttributeTest.php
tests/phpDocumentor/GraphViz/Test/AttributesTest.php
tests/phpDocumentor/GraphViz/Test/EdgeTest.php
tests/phpDocumentor/GraphViz/Test/GraphTest.php
tests/phpDocumentor/GraphViz/Test/NodeTest.php

• With this PR, The archive targets other than src/ will be as follows:

 git archive --format=tar HEAD | tar -tf - | grep -v src/
LICENSE
README.md
composer.json