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, 3 insertions, 1 deletions

diff --git a/src/nvim/getchar.h b/src/nvim/getchar.h
index 5950611d3f..be10e150e5 100644
--- a/src/nvim/getchar.h
+++ b/src/nvim/getchar.h
@@ -50,14 +50,16 @@ struct map_arguments {
 
   char_u *rhs;  /// The {rhs} of the mapping.
   size_t rhs_len;
+  LuaRef rhs_lua;  /// lua function as rhs
   bool rhs_is_noop;  /// True when the {orig_rhs} is <nop>.
 
   char_u *orig_rhs;  /// The original text of the {rhs}.
   size_t orig_rhs_len;
+  char *desc;  /// map escription
 };
 typedef struct map_arguments MapArguments;
 #define MAP_ARGUMENTS_INIT { false, false, false, false, false, false, false, \
-                             { 0 }, 0, NULL, 0, false, NULL, 0 }
+                             { 0 }, 0, NULL, 0, LUA_NOREF, false, NULL, 0, NULL }
 
 #define KEYLEN_PART_KEY -1  // keylen value for incomplete key-code
 #define KEYLEN_PART_MAP -2  // keylen value for incomplete mapping