Support autocomplete for math block

It would be nice if we support autocomplete for math block, like:
```
$$
ab*c = 123;
$$
```
when type enter after the first `$$` will autocomplete the last `$$`.

The issue describes completing a closing $$ after Enter starts a math block, but provides no file or test paths. Locate the JavaScript editor handling for Enter and math blocks, then add coverage showing that the closing delimiter is inserted and verify the example behaves as requested.

Scritto dal modello di indicizzazione a partire dal testo della issue.