feat: add support for #[builder(default)] attribute without expression

[tomoikey]

Closes #6

Summary

• Add support for bare #[builder(default)] attribute that uses Default::default()
• Maintain backward compatibility with existing #[builder(default = "expression")] syntax
• Implement DefaultValue enum to handle both cases

Changes

• Created DefaultValue enum to distinguish between bare defaults and expression defaults
• Updated parser to recognize #[builder(default)] without value using darling::FromMeta
• Modified code generation to use Default::default() for bare attributes
• Added comprehensive test coverage for new functionality

Relates to

#6