feat: update plugin with mise support

[younke]

Summary

Modernize the asdf-xcodegen plugin following the asdf-plugin-template patterns and adding mise support.

Changes

Plugin Architecture

• Add bin/download script to separate download from install (modern asdf pattern)
• Refactor lib/utils.bash with cleaner function separation (download_version, install_version)
• Add get_platform() function for future extensibility
• Update bin/install to use pre-downloaded files with fallback for older asdf versions

CI/CD Updates

• Bump asdf-vm/actions/plugin-test from v1 to v3
• Bump actions/checkout from v2 to v4

Mise Support

• Add test-mise.yml workflow to test installation via mise
• Test with XcodeGen 2.44.1 on macOS

Documentation

• Add "Test mise" badge to README

Notes

• Cross-platform (Linux) support not applicable as XcodeGen is macOS-only
• Both shellcheck and shfmt pass with no issues