feat(img): implement skeleton of vim.img.show() without backends

Implement the skeleton of `vim.img.show()` without any backend implemented.

runtime/lua/vim/img.lua

@@ -1,4 +1,5 @@
 local img = vim._defer_require('vim.img', {
+  _backend = ..., --- @module 'vim.img._backend'
   _image = ..., --- @module 'vim.img._image'
 })
 
@@ -11,4 +12,33 @@ function img.load(opts)
   return img._image:new(opts)
 end
 
+---@class vim.img.Protocol 'iterm2'|'kitty'|'sixel'
+
+---@class vim.img.Opts: vim.img.Backend.RenderOpts
+---@field backend? vim.img.Protocol|vim.img.Backend
+
+---Displays the image within the terminal used by neovim.
+---@param image vim.img.Image
+---@param opts? vim.img.Opts
+function img.show(image, opts)
+  opts = opts or {}
+
+  local backend = opts.backend
+
+  -- For named protocols, grab the appropriate backend, failing
+  -- if there is not a default backend for the specified protocol.
+  if type(backend) == 'string' then
+    local protocol = backend
+    backend = img._backend[protocol]
+    assert(backend, 'unsupported backend: ' .. protocol)
+  end
+
+  ---@cast backend vim.img.Backend
+  backend.render(image, {
+    pos = opts.pos,
+    size = opts.size,
+    crop = opts.crop,
+  })
+end
+
 return img

runtime/lua/vim/img/_backend.lua

@@ -0,0 +1,10 @@
+---@class vim.img.Backend
+---@field render fun(image:vim.img.Image, opts?:vim.img.Backend.RenderOpts)
+
+---@class vim.img.Backend.RenderOpts
+---@field crop? {x:integer, y:integer, width:integer, height:integer} units are pixels
+---@field pos? {row:integer, col:integer} units are cells
+---@field size? {width:integer, height:integer} units are cells
+
+return {
+}

runtime/lua/vim/img/_image.lua

@@ -33,6 +33,12 @@ function M:size()
   return string.len(self.data or '')
 end
 
+---Displays the image within the terminal used by neovim.
+---@param opts? vim.img.Opts
+function M:show(opts)
+  vim.img.show(self, opts)
+end
+
 ---Loads data for an image from a file, replacing any existing data.
 ---If a callback provided, will load asynchronously; otherwise, is blocking.
 ---@param filename string