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/api/buffer.c

@@ -835,7 +835,7 @@ Integer nvim_buf_get_changedtick(Buffer buffer, Error *err)
 /// @param[out]  err   Error details, if any
 /// @returns Array of maparg()-like dictionaries describing mappings.
 ///          The "buffer" key holds the associated buffer handle.
-ArrayOf(Dictionary) nvim_buf_get_keymap(Buffer buffer, String mode, Error *err)
+ArrayOf(Dictionary) nvim_buf_get_keymap(uint64_t channel_id, Buffer buffer, String mode, Error *err)
   FUNC_API_SINCE(3)
 {
   buf_T *buf = find_buffer_by_handle(buffer, err);
@@ -844,7 +844,7 @@ ArrayOf(Dictionary) nvim_buf_get_keymap(Buffer buffer, String mode, Error *err)
     return (Array)ARRAY_DICT_INIT;
   }
 
-  return keymap_array(mode, buf);
+  return keymap_array(mode, buf, channel_id == LUA_INTERNAL_CALL);
 }
 
 /// Sets a buffer-local |mapping| for the given mode.