feat: implement \relax as no-op function (#3384)

* feat: implement \relax as no-op function

BREAKING CHANGE: `\relax` is now implemented as a function. It'll stop
expansions and parsing, so the behavior around `\relax` may change.
For example, `\kern2\relax em` will no longer work.

test/katex-spec.js

@@ -4046,3 +4046,9 @@ describe("debugging macros", () => {
         });
     });
 });
+
+describe("\\relax", () => {
+    it("should stop the expansion", () => {
+        expect`\kern2\relax em`.not.toParse();
+    });
+});