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`.

1 files changed, 8 insertions, 2 deletions

diff --git a/src/nvim/eval/funcs.c b/src/nvim/eval/funcs.c
index 32026282cf..4cccecb23a 100644
--- a/src/nvim/eval/funcs.c
+++ b/src/nvim/eval/funcs.c
@@ -5980,6 +5980,7 @@ static void get_maparg(typval_T *argvars, typval_T *rettv, int exact)
 {
   char_u *keys_buf = NULL;
   char_u *rhs;
+  LuaRef rhs_lua;
   int mode;
   int abbr = FALSE;
   int get_dict = FALSE;
@@ -6016,7 +6017,7 @@ static void get_maparg(typval_T *argvars, typval_T *rettv, int exact)
 
   keys = replace_termcodes(keys, STRLEN(keys), &keys_buf, true, true, true,
                            CPO_TO_CPO_FLAGS);
-  rhs = check_map(keys, mode, exact, false, abbr, &mp, &buffer_local);
+  rhs = check_map(keys, mode, exact, false, abbr, &mp, &buffer_local, &rhs_lua);
   xfree(keys_buf);
 
   if (!get_dict) {
@@ -6027,10 +6028,15 @@ static void get_maparg(typval_T *argvars, typval_T *rettv, int exact)
       } else {
         rettv->vval.v_string = (char_u *)str2special_save((char *)rhs, false, false);
       }
+    } else if (rhs_lua != LUA_NOREF) {
+      size_t msglen = 100;
+      char *msg = (char *)xmalloc(msglen);
+      snprintf(msg, msglen, "<Lua function %d>", mp->m_luaref);
+      rettv->vval.v_string = (char_u *)msg;
     }
   } else {
     tv_dict_alloc_ret(rettv);
-    if (rhs != NULL) {
+    if (mp != NULL && (rhs != NULL || rhs_lua != LUA_NOREF)) {
       // Return a dictionary.
       mapblock_fill_dict(rettv->vval.v_dict, mp, buffer_local, true);
     }