Support vim's func[line_num] format for callstacks

I still use this plugin a lot and it's fantastic, but I'd love if I could use it to jump to vim callstacks in the quickfix. When you get a vim error, you can use vim-scriptease's :Messages to view the errors in the quickfix. It displays them like this:

 Error in function
 dirvish#open[39]
 <SNR>569_open_dir[31]
 E472: Command failed

It'd be amazing to call lookup() on dirvish#open, have it recognize it's a function name, that the 39 means the 39th line in that function, and to jump to that line within the function.

You can use :put =execute('messages') to dump messages into a buffer without a plugin, and the func[line_num] format is a vim feature. So maybe it's in scope for vim-lookup to support? If not, maybe it's an argument for #15.

Start at the lookup() entry point and inspect how it identifies Vim function names. Use the func[line_num] examples from :Messages, including dirvish#open[39] and 569_open_dir[31], to define the expected jump behavior; done means lookup() resolves the function and jumps to the specified line.