Update .gitattributes to not package unrequired files in releases (#272)

Author: ben-challis

.gitattributes

@@ -1 +1,11 @@
-/composer.lock      -merge
+/.github                export-ignore
+/tests                  export-ignore
+.gitattributes          export-ignore
+.gitignore              export-ignore
+.php-cs-fixer.php       export-ignore
+composer.lock           export-ignore -merge
+infection.json.dist     export-ignore
+phpstan.neon            export-ignore
+phpstan-rector.neon     export-ignore
+phpunit.xml.dist        export-ignore
+rector.php              export-ignore