Feature development: Allow reveal.js configuration from notebook metadata with correct precedence

[bouzidanas]

This request introduces a flexible configuration system for reveal.js slides, allowing users to pass a full configuration dictionary directly from notebook metadata.

Previously, only a limited set of reveal.js options were made accessible via the command line and the configuration file. Now, any option can be set via a reveal key in the notebook's metadata, which is passed directly to Reveal.initialize().

This change also establishes and enforces a clear order of configuration precedence (override order):

1. Command-line arguments (highest)
2. Notebook metadata (metadata.reveal)
3. Config file (SlidesExporter.reveal_config)
4. Hard-coded defaults (lowest)

Key improvements include:

• A new Dict traitlet for config file settings.
• Updated logic to correctly layer configurations.
• A robust check for command-line arguments, fixing previous AttributeError bugs.
• The index.html.j2 template is now enhanced to safely merge the dynamic configuration, preserving essential default values and preventing empty metadata from overriding them.