feat: configurable explorer position (left or bottom)

[esmuellert]

Summary

Adds configurable explorer panel position, allowing users to place the file list at the bottom instead of left for a wider diff view.

Changes

• Add explorer.position config option: 'left' (default) or 'bottom'
• Add explorer.width config option for left position (default: 40)
• Add explorer.height config option for bottom position (default: 15)
• Document diff_get and diff_put keymaps in README

Usage

require('vscode-diff').setup({
  explorer = {
    position = 'bottom',  -- Place file list at bottom
    height = 15,          -- Height in lines
  },
})

Closes #103