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/buffer_defs.h

@@ -352,6 +352,7 @@ struct mapblock {
   char_u *m_keys;          // mapped from, lhs
   char_u *m_str;           // mapped to, rhs
   char_u *m_orig_str;      // rhs as entered by the user
+  LuaRef m_luaref;         // lua function reference as rhs
   int m_keylen;                 // strlen(m_keys)
   int m_mode;                   // valid mode
   int m_noremap;                // if non-zero no re-mapping for m_str
@@ -359,6 +360,7 @@ struct mapblock {
   char m_nowait;                // <nowait> used
   char m_expr;                  // <expr> used, m_str is an expression
   sctx_T m_script_ctx;          // SCTX where map was defined
+  char *m_desc;                 // description of keymap
 };
 
 /// Used for highlighting in the status line.