[Req] Highlisted text should be pasted into prompt when openChat is called

Often times you want to ask a question about a piece of code in the editor.

It would be nice to transparently paste in any highlighted text, possible including the full file, into ChatGPT (given that the user doesn't mind about token count).

I think ChatGPT is smart enough to handle a message like this

```
{some snippet of code}
---
The above code (continue prompt...)
```

Start at the openChat entry point mentioned in the issue and trace how the current prompt is assembled. Check how highlighted text or the full editor file can be obtained, then verify that selected content is included before the continuation prompt when openChat is called.

Written by the indexing model from the issue text.