feature: Ignore ranges

[pawamoy]

Is your feature request related to a problem? Please describe.

With a consequent block of lines to ignore, it's tedious to add a markdown-exec: hide comment to all these lines.

Describe the solution you'd like

Two new markers to mark the start and beginning of entire blocks to ignore:

```python exec="on"
# markdown-exec: hide-start
...
...  # these lines won't be shown
...
# markdown-exec: hide-end
print("...")
```

Describe alternatives you've considered

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Additional context

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