Wanted to propose a code structure change, but I understand if you're partial to your correct organization. This PR does the following:

• Introduces prettier for automated code formatting
• Moves tool registration to their own files (e.g. tools/docs/addComment.ts). Idea here was to make it easier for developers to quickly see what tools are supported, what domain they are in (e.g. docs vs sheets), and how they can contribute new ones.
• Adds support for horizontal rule tags in markdown conversion (I just saw this gap and wanted to fix it)
• Fixes a test that was broken

Feel free to reject this if you don't want changes like this. Just thought I'd offer.