feat: add original_position option to control diff layout

[esmuellert]

Summary

Add diff.original_position config option to control where the original (old) content appears in the diff view.

Changes

• Add original_position option in config.lua with values "left" (default) or "right"
• Modify view/init.lua to use leftabove vsplit when original_position = "right"
• Update README documentation

Usage

require("codediff").setup({
  diff = {
    original_position = "right",  -- Modified on left, original on right
  },
})

Testing

All existing tests pass.