feat: support \angl and \angln (#2334)

* Support \angl and \angln

* Update screenshots

* Update documentation

test/katex-spec.js

@@ -2465,6 +2465,24 @@ describe("A strike-through builder", function() {
     });
 });
 
+describe("A actuarial angle parser", function() {
+    it("should not fail in math mode", function() {
+        expect`a_{\angl{n}}`.toParse();
+    });
+    it("should fail in text mode", function() {
+        expect`\text{a_{\angl{n}}}`.not.toParse();
+    });
+});
+
+describe("A actuarial angle builder", function() {
+    it("should not fail", function() {
+        expect`a_{\angl{n}}`.toBuild();
+        expect`a_{\angl{n}i}`.toBuild();
+        expect`a_{\angl n}`.toBuild();
+        expect`a_\angln`.toBuild();
+    });
+});
+
 describe("A phantom parser", function() {
     it("should not fail", function() {
         expect`\phantom{x}`.toParse();