feat: add code check

[peter-jerry-ye]

No description provided.

[bobzhang]

I made a syntax error but did not find error reported, did I miss anything

[peter-jerry-ye]

@bobzhang you need to write explicitly like:

```{code-block} moonbit
:class: top-level
```

or

```{code-block} moonbit
:class: expr
```

otherwise I can't distinguish with the already checked code.

[bobzhang]

can you double check? it does not work for me, no error message found

[peter-jerry-ye]

Should be fixed now.