Update type hint so luals provides completion for available image providers

Author: dangooddd

lua/pyrepl/types.lua

@@ -24,7 +24,7 @@
 ---@field image_max_history? integer
 ---@field image_width_ratio? number
 ---@field image_height_ratio? number
----@field image_provider? string
+---@field image_provider? "placeholders" | "image" | string
 ---@field cell_pattern? string
 ---@field python_path? string
 ---@field preferred_kernel? string