fix: detach open command execution (#80)

This commit simply switches `os.execute` with `vim.fn.jobstart` with
`detach` set to `true`, making the editor available after opening the
problems' url allowing you to edit, while the browser is running.

The `detach` option keeps the browser open after the editor shuts down.
It is not the default behavior (which would make the browser shut down
with the editor, I find the introduced behavior to be more appropriate).

NOTE: This commit has been tested only on Linux; but I don't suppose
there would be any meaningful difference.

lua/leetcode/command/init.lua

@@ -313,7 +313,7 @@ function cmd.open()
  command = string.format("start \"\" \"%s\"", q.cache.link)
  end
 
- os.execute(command)
+ vim.fn.jobstart(command, { detach = true })
  end
 end