feat(slides): Allow reveal.js configuration from notebook metadata

[bouzidanas]

Description

This pull request introduces a new feature that allows reveal.js presentation settings to be configured directly within a notebook's metadata.

Currently, reveal.js options for slide exports are primarily set via command-line arguments or configuration files. This separates the presentation's configuration from its content, making it less portable. For example, if a user wants to share a notebook that is designed to be presented with a specific theme or transition, they must also share the command-line flags needed to reproduce it.

This enhancement allows users to embed reveal.js settings within the .ipynb file itself, making presentations self-contained and easier to share and reproduce consistently.

Implementation Details

The feature is implemented by overriding the from_notebook_node method in the SlidesExporter. The new implementation preserves the existing conversion logic by first calling super().from_notebook_node(). It then inspects the notebook's metadata for a reveal key and merges the settings with the configuration already loaded from command-line options and defaults.

A clear order of precedence is maintained to ensure intuitive and predictable behavior:

1. Command-line arguments (e.g., --SlidesExporter.reveal_theme=night) have the highest priority.
2. Notebook metadata is applied next.
3. Exporter defaults are used as the base (lowest priority).

This ensures that users can set project-wide defaults in a notebook but still easily override them from the command line for a specific export.

How to Use

To use this feature, a user can edit the notebook's metadata (e.g., in Jupyter, via Edit > Edit Notebook Metadata) and add a reveal object.

Example Notebook Metadata:

{
  "kernelspec": {
    "display_name": "Python 3",
    "language": "python",
    "name": "python3"
  },
  "language_info": {
    "name": "python",
    "version": "3.9.7"
  },
  "reveal": {
    "theme": "sky",
    "transition": "zoom",
    "scroll": true
  }
}

When a notebook with this metadata is exported using jupyter nbconvert --to slides, it will automatically use the "sky" theme and "zoom" transition, unless a different value is provided on the command line.

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