Make Problem.fix() internal (#7785)

We cannot publish this feature because we're still discussing the signature etc.

types/stylelint/index.d.ts

@@ -630,6 +630,10 @@ declare namespace stylelint {
 		 * Optional severity override for the problem.
 		 */
 		severity?: RuleSeverity;
+		/**
+		 * @internal
+		 * WARNING: Don't use this feature now. It may change in the future.
+		 */
 		fix?: () => void | never | Range;
 	};