module: simpler shebang function

This simplifies the shebang function significantly. Before, it was
optimized for two characters input. Any module actually parsed should
however have more characters than just the shebang.
The performance stays the same as before.

PR-URL: https://github.com/nodejs/node/pull/26266


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>