Add theme installer and uninstall command for Hexo

[FloranceYeh]

check list

• Add test cases for the changes.
• Passed the CI test.

What does it do?

This PR adds theme management commands to Hexo CLI:

• Add hexo theme install <theme-name>

  • Find themes from the official Hexo theme registry

  • Clone the theme into themes/<theme-name>

  • Install production dependencies using the site's package manager

  • Copy the theme's _config.yml to _config.<theme-name>.yml

  • Preserve an existing root theme configuration

  • Add hexo theme uninstall <theme-name>

    • Remove the installed theme directory
    • Remove the corresponding _config.<theme-name>.yml
    • Avoid accessing the remote registry during uninstall

  No additional runtime dependencies are introduced.

Tests

• Added tests for theme lookup and installation
• Added tests for dependency installation and package manager selection
• Added tests for theme configuration copying
• Added tests for uninstall behavior and failure cases
• Verified with npm test — 66 passing