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, 6 insertions, 3 deletions

diff --git a/src/nvim/api/vim.c b/src/nvim/api/vim.c
index dfc606f927..693d2083e6 100644
--- a/src/nvim/api/vim.c
+++ b/src/nvim/api/vim.c
@@ -1538,10 +1538,10 @@ Dictionary nvim_get_mode(void)
 /// @param  mode       Mode short-name ("n", "i", "v", ...)
 /// @returns Array of maparg()-like dictionaries describing mappings.
 ///          The "buffer" key is always zero.
-ArrayOf(Dictionary) nvim_get_keymap(String mode)
+ArrayOf(Dictionary) nvim_get_keymap(uint64_t channel_id, String mode)
   FUNC_API_SINCE(3)
 {
-  return keymap_array(mode, NULL);
+  return keymap_array(mode, NULL, channel_id == LUA_INTERNAL_CALL);
 }
 
 /// Sets a global |mapping| for the given mode.
@@ -1566,7 +1566,10 @@ ArrayOf(Dictionary) nvim_get_keymap(String mode)
 /// @param  lhs   Left-hand-side |{lhs}| of the mapping.
 /// @param  rhs   Right-hand-side |{rhs}| of the mapping.
 /// @param  opts  Optional parameters map. Accepts all |:map-arguments|
-///               as keys excluding |<buffer>| but including |noremap|.
+///               as keys excluding |<buffer>| but including |noremap| and "desc".
+///               |desc| can be used to give a description to keymap.
+///               When called from Lua, also accepts a "callback" key that takes
+///               a Lua function to call when the mapping is executed.
 ///               Values are Booleans. Unknown key is an error.
 /// @param[out]   err   Error details, if any.
 void nvim_set_keymap(String mode, String lhs, String rhs, Dict(keymap) *opts, Error *err)