deprecate: input.prompt_buffer option

Even once the fixes to the prompt buffer come out in the next version of
Neovim, I'm really not sure what advantages we get from using the prompt
buffer. And we will continue to have disadvantages because it does *not*
function like a normal buffer. If anyone is using it and has a good
reason for it, please let me know. Otherwise, I'll be removing that
functionality in a later commit so I don't have to support two
implementations.

README.md

@@ -138,9 +138,6 @@ require('dressing').setup({
     -- Window transparency (0-100)
     winblend = 10,
 
-    -- see :help dressing-prompt
-    prompt_buffer = false,
-
     -- see :help dressing_get_config
     get_config = nil,
   },

doc/dressing.txt

@@ -28,9 +28,6 @@ Configure dressing.nvim by calling the setup() function.
         -- Window transparency (0-100)
         winblend = 10,
     
-        -- see :help dressing-prompt
-        prompt_buffer = false,
-    
         -- see :help dressing_get_config
         get_config = nil,
       },

lua/dressing/config.lua

@@ -21,9 +21,6 @@ local default_config = {
     -- Window transparency (0-100)
     winblend = 10,
 
-    -- see :help dressing-prompt
-    prompt_buffer = false,
-
     -- see :help dressing_get_config
     get_config = nil,
   },

lua/dressing/init.lua

@@ -4,6 +4,16 @@ local M = {}
 
 M.setup = function(opts)
   config.update(opts)
+  -- Wait until vim has fully started up before we show deprecation warnings
+  vim.defer_fn(function()
+    if config.input.prompt_buffer then
+      vim.notify(
+        "dressing.nvim option 'input.prompt_buffer = true' is deprecated and will be removed in a future version. If you want it to continue to be supported please file an issue with your use case: https://github.com/stevearc/dressing.nvim/issues/new",
+        vim.log.levels.WARN,
+        {}
+      )
+    end
+  end, 100)
 end
 
 local original_input