Currently `ChatOutputParser` extracts actions by splitting the text on
"```", and then load the second part as a json string.
But sometimes the LLM will wrap the action in markdown code block like:
````markdown
```json
{
"action": "foo",
"action_input": "bar"
}
```
````
Splitting text on "```" will cause `OutputParserException` in such case.
This PR changes the behaviour to extract the `$JSON_BLOB` by regex, so
that it can handle both ` ``` ``` ` and ` ```json ``` `
@hinthornw
---------
Co-authored-by: Junlin Zhou <jlzhou@zjuici.com>
