Composer: add script descriptions

These descriptions will be used when a list of the available scripts is requested on the command-line using the `composer list` or `composer run -l` commands.

These descriptions also help document the different scripts for the maintainers of the `composer.json` file.

Ref: https://getcomposer.org/doc/articles/scripts.md#custom-descriptions-

composer.json

@@ -76,5 +76,16 @@
 		"integration-coverage": [
 			"@coverage-wp"
 		]
+	},
+	"scripts-descriptions": {
+		"lint": "Check the PHP files for parse errors.",
+		"check-cs": "Check the PHP files for code style violations and best practices.",
+		"fix-cs": "Auto-fix code style violations in the PHP files.",
+		"test": "Run the unit tests without code coverage.",
+		"coverage": "Run the unit tests with code coverage.",
+		"test-wp": "Run the WP unit tests without code coverage.",
+		"coverage-wp": "Run the WP unit tests with code coverage.",
+		"integration-test": "Deprecated. Alias for the \"test-wp\" script.",
+		"integration-coverage": "Deprecated. Alias for the \"coverage-wp\" script."
 	}
 }