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`.
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