Add \yen support to text mode (#1208)

Fix #1204 by adding `\yen` support to text mode.

More generally, I checked all the text+mode symbols supported by
[amsfonts](http://mirrors.ctan.org/fonts/amsfonts/doc/amsfonts.pdf)
(see Section 6), and found that `\yen` is the only one lacking text-mode
support.

test/katex-spec.js

@@ -2612,6 +2612,16 @@ describe("The symbol table integrity", function() {
     });
 });
 
+describe("Symbols", function() {
+    it("should support AMS symbols in both text and math mode", function() {
+        // These text+math symbols are from Section 6 of
+        // http://mirrors.ctan.org/fonts/amsfonts/doc/amsfonts.pdf
+        const symbols = "\\yen\\checkmark\\circledR\\maltese";
+        expect(symbols).toBuild();
+        expect(`\\text{${symbols}}`).toBuild();
+    });
+});
+
 describe("A macro expander", function() {
 
     const compareParseTree = function(actual, expected, macros) {