refactor: mdx rehype plugin

[peppermintc]

Summary

• Refactor rehype plugin, tried to improve maintainability
• Test available with this PR
• Tried to remain same functionalities, but if possible cross check would be nice

File Structure

src/
 ├─ index.ts          // plugin entry
 ├─ core.ts           // load options and visit hast
 ├─ handlers.ts       // handle pre code node logic
 ├─ utils.ts          // helper functions
 ├─ types.ts          // types
 ├─ constants.ts      // constants

Comments

• As inspecting plugin logic, I thought maintainability could improve on this plugin part. So while understanding the code I made some changes
• If this refactor doesn’t align with the project direction, please feel free to close this PR