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/normal.c

@@ -334,6 +334,7 @@ static const struct nv_cmd {
   { K_SELECT,  nv_select,      0,                      0 },
   { K_EVENT,   nv_event,       NV_KEEPREG,             0 },
   { K_COMMAND, nv_colon,       0,                      0 },
+  { K_LUA, nv_colon,           0,                      0 },
 };
 
 // Number of commands in nv_cmds[].
@@ -4043,21 +4044,22 @@ static void nv_regreplay(cmdarg_T *cap)
   }
 }
 
-/// Handle a ":" command and <Cmd>.
+/// Handle a ":" command and <Cmd> or Lua keymaps.
 static void nv_colon(cmdarg_T *cap)
 {
   int old_p_im;
   bool cmd_result;
   bool is_cmdkey = cap->cmdchar == K_COMMAND;
+  bool is_lua = cap->cmdchar == K_LUA;
 
-  if (VIsual_active && !is_cmdkey) {
+  if (VIsual_active && !is_cmdkey && !is_lua) {
     nv_operator(cap);
   } else {
     if (cap->oap->op_type != OP_NOP) {
       // Using ":" as a movement is charwise exclusive.
       cap->oap->motion_type = kMTCharWise;
       cap->oap->inclusive = false;
-    } else if (cap->count0 && !is_cmdkey) {
+    } else if (cap->count0 && !is_cmdkey && !is_lua) {
       // translate "count:" into ":.,.+(count - 1)"
       stuffcharReadbuff('.');
       if (cap->count0 > 1) {
@@ -4073,9 +4075,13 @@ static void nv_colon(cmdarg_T *cap)
 
     old_p_im = p_im;
 
+    if (is_lua) {
+      cmd_result = map_execute_lua();
+    } else {
     // get a command line and execute it
-    cmd_result = do_cmdline(NULL, is_cmdkey ? getcmdkeycmd : getexline, NULL,
-                            cap->oap->op_type != OP_NOP ? DOCMD_KEEPLINE : 0);
+      cmd_result = do_cmdline(NULL, is_cmdkey ? getcmdkeycmd : getexline, NULL,
+                              cap->oap->op_type != OP_NOP ? DOCMD_KEEPLINE : 0);
+    }
 
     // If 'insertmode' changed, enter or exit Insert mode
     if (p_im != old_p_im) {