Add format-ipy-cells builtin hook

Add a Rust-native builtin hook that formats `# %%` cell delimiters in
VS Code interactive Python notebooks. Based on the Python
[format-ipy-cells](https://github.com/janosh/format-ipy-cells) hook,
reimplemented using a structured line-based parser instead of chained
regex substitutions.

The hook normalizes cell delimiter spacing, comment formatting, removes
empty cells, ensures consistent blank lines between cells, and handles
module docstring spacing.

Usage:
```yaml
repos:
  - repo: builtin
    hooks:
      - id: format-ipy-cells
```

Author: janosh