WIP: detect js by shebang and run by filetype

[coolaj86]

Re: #340

Summary

Although these same lines work for me in my local config, This doesn't work directly in the vim-prettier plugin.

However, for someone who actually knows vimscript and how plugins work, I imagine understanding the intent and making the change for it to work will be trivial.

Test Plan

1. Create the test file ./test-no-extension with these contents:
   ./test-no-extension

   #!/usr/bin/env node
   
   function main() {
   console.log('Hello');
   }

2. Open the file

   vim ./test-no-extension

3. Save the file

   :w

If it auto-formats, it worked.