feat: add null option and rewrite README

[fahchen]

Summary

• Add null option for field types to control nullability explicitly
• Rewrite README for clarity and conversational tone

Changes

New null option

Fields can now use null: false to prevent nil from being added to their type, or null: true (default) to allow it. This is useful for database records where some fields are always present (like id, inserted_at).

typed_structor do
  field :id, integer(), null: false       # Never nil
  field :name, String.t(), null: true     # Always nullable
end

The option can also be set at the module level:

typed_structor null: false do
  field :id, integer()                    # Not nullable by default
  field :phone, String.t(), null: true    # Override for this field
end

README rewrite

• Added "Why TypedStructor?" section with before/after comparison
• Reorganized content into logical sections (Getting Started, Options, Common Patterns, Advanced Features)
• Made examples more practical and self-contained
• Improved formatting with callouts and tables

Changelog

- feat: add null option for field types
- docs: rewrite README for clarity and conversational tone

[Copilot]

Pull request overview

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