feat(api): add support for lua function & description in keymap

Behavioral changes:

1. Added support for lua function in keymaps in
--------------------------------------------
- nvim_set_keymap
  Can set lua function as keymap rhs like following:
```lua
vim.api.nvim_{buf_}set_keymap('n', '<leader>lr', '', {callback = vim.lsp.buf.references})
```
  Note: lua function can only be set from lua . If api function being
  called from viml or over rpc this option isn't available.
- nvim_{buf_}get_keymap
  When called from lua, lua function is returned is `callback` key .
  But in other cases callback contains number of the function ref.
- :umap, nvim_del_keymap & nvim_buf_del_keymap clears lua keymaps correctly.
- :map commands for displaing rhs .
   For lua keymaps rhs is displayed as <Lua function ref_no>
   Note: lua keymap cannot be set through viml command / functions.
- mapargs()
  When dict is false it returns string in `<Lua function ref_no>`
  format (same format as :map commands).
  When dict is true it returns ref_no number in `callback` key.
- mapcheck()
  returns string in `<Lua function ref_no>` format (same format as :map commands).

2. Added support for keymap description
---------------------------------------
- nvim_{buf_}set_keymap: added `desc` option in opts table .
 ```lua
vim.api.nvim_set_keymap('n', '<leader>w', '<cmd>w<cr>', {desc='Save current file'})
```
- nvim_{buf_}get_keymap: contains `desc` in returned list.
- commands like `:nmap <leader>w` will show description in a new line below rhs.
- `maparg()` return dict contains `desc`.

src/nvim/eval.c

@@ -7299,12 +7299,19 @@ void mapblock_fill_dict(dict_T *const dict, const mapblock_T *const mp, long buf
     noremap_value = mp->m_noremap == REMAP_SCRIPT ? 2 : !!mp->m_noremap;
   }
 
-  if (compatible) {
-    tv_dict_add_str(dict, S_LEN("rhs"), (const char *)mp->m_orig_str);
+  if (mp->m_luaref != LUA_NOREF) {
+    tv_dict_add_nr(dict, S_LEN("callback"), mp->m_luaref);
   } else {
-    tv_dict_add_allocated_str(dict, S_LEN("rhs"),
-                              str2special_save((const char *)mp->m_str, false,
-                                               true));
+    if (compatible) {
+      tv_dict_add_str(dict, S_LEN("rhs"), (const char *)mp->m_orig_str);
+    } else {
+      tv_dict_add_allocated_str(dict, S_LEN("rhs"),
+                                str2special_save((const char *)mp->m_str, false,
+                                                 true));
+    }
+  }
+  if (mp->m_desc != NULL) {
+    tv_dict_add_allocated_str(dict, S_LEN("desc"), xstrdup(mp->m_desc));
   }
   tv_dict_add_allocated_str(dict, S_LEN("lhs"), lhs);
   tv_dict_add_nr(dict, S_LEN("noremap"), noremap_value);