Add input/output parameter syntax for explicit dependency management

[artste]

This PR introduces a powerful new feature for %%testcell that enables explicit control over which symbols enter and exit isolated cells using the (inputs)->(outputs) syntax.

Additionally, the README.md has been completely rewritten from scratch, and a new notebook testcell_zoo.ipynb has been added, showcasing real-world use cases.

Main Feature

Explicit input/output syntax for %%testcelln:

• Syntax: %%testcelln (inputs)->(outputs)
• Control: Specify exactly which symbols are accessible in the cell and which should be saved back to globals
• Benefits: Forces explicit dependency declaration, preventing accidental reliance on global state

%%testcelln (data)->(calculate_stats)
# Only 'data' is available, only 'calculate_stats' is saved
def calculate_stats(values):
    return {'mean': sum(values) / len(values)}

Implementation

• New module: testcell/inout.py - Handles parsing and processing of input/output parameters
• Updated core: Enhanced testcell/init.py with inout parameter support
• New documentation: nbs/01a_arguments.ipynb - Comprehensive guide to the new syntax

Documentation & Examples

• Completely rewritten README: Clearer structure, better examples, focuses on practical use cases
• New demo notebook: demo/testcell_zoo.ipynb - Extensive collection of real-world use cases
• Enhanced demo: Updated demo/testcell_demo.ipynb with inout parameter examples and cells output
• Improved rendering: Switched from HTML to Markdown code blocks for better cross-platform compatibility

Additional Improvements

• Fixed code block rendering (removed extra newlines)
• Updated badge examples
• Fixed typos throughout documentation
• Refactored nbs/index.ipynb for better organization
• Improved visual styling for skipped cells
• Added "Open in Modal" badge
• Fixed HTML/XSS vulnerability in MessageBox class (added html.escape())
• Improved error messages in validate_and_update_inputs