patch 8.2.1054: not so easy to pass a lua function to Vim

Problem:    Not so easy to pass a lua function to Vim.
Solution:   Convert a Lua function and closure to a Vim funcref. (Prabir
            Shrestha, closes #6246)

runtime/doc/if_lua.txt

@@ -333,6 +333,14 @@ Examples:
 	:lua l = d.len -- assign d as 'self'
 	:lua print(l())
 <
+Lua functions and closures are automatically converted to a Vim |Funcref| and
+can be accessed in Vim scripts.  Example:
+>
+	lua <<EOF
+	vim.fn.timer_start(1000, function(timer)
+	    print('timer callback')
+	end)
+	EOF
 
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 7. Buffer userdata					*lua-buffer*