feat: add alt-diffview option

[KEY60228]

Summary

Added support for alt-diffview and improved untracked file display.

Changes

• Added "alt-diffview" as an option for diff_tool configuration
• Implemented alt-diffview adapter (lua/reviewthem/diff/alt_diffview.lua)
• Fixed untracked files not being displayed when comparing refs with working tree
• Updated documentation to recommend alt-diffview for specific use cases

Motivation

When comparing a specific ref with the working tree (e.g., ReviewThemStart main), diffview.nvim doesn't properly display untracked files. alt-diffview handles this scenario more reliably, so we added support for it.

Technical Details

1. Untracked file detection: Using git ls-files --others --exclude-standard to detect untracked files and include them in the diff list
2. Diff tool abstraction: Managing diff tool switching through lua/reviewthem/diff/init.lua
3. URI handling: Supporting different URI formats between alt-diffview and diffview.nvim

Testing

• Verified untracked files are displayed with ReviewThemStart
• Verified untracked files are displayed with ReviewThemStart main
• Tested with both alt-diffview and diffview.nvim
• Confirmed health check works correctly

Notes

• For comparing refs with working tree, we recommend setting diff_tool = "alt-diffview" as documented in the README
• Appropriate error messages are displayed when neither diff tool is installed