feat: add .gitattributes to exclude unnecessary files from Composer dist package (#67)

Author: maartenpaauw

.gitattributes

@@ -0,0 +1,11 @@
+# Directories
+/.github export-ignore
+/tests   export-ignore
+
+# Files
+/.editorconfig            export-ignore
+/.gitattributes           export-ignore
+/.gitignore               export-ignore
+/.php-cs-fixer.php        export-ignore
+/block-kit-screenshot.png export-ignore
+/phpstan.neon             export-ignore