Implicitly create `extension_dir` if it does not exist

[asgrim]

Currently \Php\Pie\Platform\TargetPhp\PhpBinaryPath::extensionPath will fail if the extension_dir is set, but it does not exist.

After some discussion, it seems either pecl or brew would create this path if it doesn't exist.

So, we COULD attempt to create this directory. I need to think to make sure it's the right approach.