FIX: ensures chat is present/enabled (#68)

Without this when chat is disabled the API is not supercharged with these methods and it will cause an exception:

```
Uncaught (in promise) TypeError: t.decorateChatMessage is not a function
    initialize discourse-math-mathjax.js:102
    initialize discourse-math-mathjax.js:119
    withPluginApi discourse-f7231bd5d10aa613b323a0cac71d8003baa74ef74f674fe4f91e4857994e661b.br.js:4735
    initialize discourse-math-mathjax.js:118
    initialize discourse-f7231bd5d10aa613b323a0cac71d8003baa74ef74f674fe4f91e4857994e661b.br.js:67
    runInstanceInitializers Ember
    each dag-map.js:192
    walk dag-map.js:121
    each dag-map.js:66
    topsort dag-map.js:72
    Ember 4
    invoke queue.ts:201
    flush queue.ts:98
    flush deferred-action-queues.ts:75
    _end index.ts:616
    _boundAutorunEnd index.ts:257
discourse-math-mathjax.js:102
```

assets/javascripts/initializers/discourse-math-mathjax.js

@@ -116,6 +116,7 @@ function initializeMath(api, discourseMathOptions) {
     { id: "mathjax" }
   );
 
+  if (api.decorateChatMessage) {
     api.decorateChatMessage(
       (element) => {
         mathjax(element, discourseMathOptions);
@@ -124,6 +125,7 @@ function initializeMath(api, discourseMathOptions) {
         id: "mathjax-chat",
       }
     );
+  }
 }
 
 export default {