Add .gitattributes for export-ignore and diff customization

This commit introduces a .gitattributes file to exclude unnecessary files from being included in archives, such as build and test files. Additionally, it configures custom diff rules for .php and .phar files to improve the clarity of diffs.

.gitattributes

@@ -0,0 +1,17 @@
+# Exclude build/test files from archive.
+/tests                           export-ignore
+/.gitattributes                  export-ignore
+/.gitignore                      export-ignore
+/.scrutinizer.yml                export-ignore
+/php-require-checker.config.json export-ignore
+/php.ini                         export-ignore
+/codecov.yml                     export-ignore
+/phpcs.xml                       export-ignore
+/phpmd.xml                       export-ignore
+/phpstan.neon                    export-ignore
+/psalm.xml                       export-ignore
+/phpunit.xml.dist                export-ignore
+
+# Configure diff output for .php and .phar files.
+*.php diff=php
+*.phar -diff