[PHP utils] Export splitShellCommand

[adamziel]

Motivation for the change, related issues

Pre-requisite for #2699. Empowers developers to create terminal-enabled apps by exporting the splitShellCommand() utility that was internal before.

import { splitShellCommand } from "@php-wasm/util";
splitShellCommand('wp create post --title="Great post title!"');
['wp', 'create', 'post', '--title=Great post title!']