feat(api): support abbreviations in nvim_set_keymap

closes #19198

src/nvim/api/vim.c

@@ -1420,6 +1420,7 @@ ArrayOf(Dictionary) nvim_get_keymap(String mode)
 /// @param channel_id
 /// @param  mode  Mode short-name (map command prefix: "n", "i", "v", "x", …)
 ///               or "!" for |:map!|, or empty string for |:map|.
+///               "ia", "ca" or "!a" for abbreviation in insert mode, cmdline mode, or both, respectively
 /// @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 except |<buffer>|,