Create cache directory if it does not exist about phpsyllable

[Blackskyliner]

Problem here is: If we create the cache in production code (aka the Syllable class) we have a problem if used with Composer, because if you would not change the Cache-Dir outside of the preconfigured __DIR__ you would have cache files created within the vendor directory. (where it strictly does not belong)

This may or may not be a "problem" if you have shared vendors across same-plattform multiple-instances Projects which would use syllable w/o manually configuring the cache dir per instance.

Tests on master already will create a cache, but I would vote not to automatically create directories within the class and thus in productive code itself. We could however throw a RuntimeException to notify the User to create or configure a cache directory.

I guess the real alternative would be not to auto-wire dependencies itself within its own constructor.
This would lead to a decrease in Developer Experience as there would be more to configure initially, but then we could have the Cacheing Component have a auto-create-if-not-exists logic as there would be no DIR default anymore that would point into the vendor dir.

Though the cache adapter used by Syllable could just adhere to PSR-6 (https://www.php-fig.org/psr/psr-6/) whereas we could just use already existing cache providers, even those not living in the Filesystem but inside Cloud or Memory Adapters, which in return will already create the necessary structure if not already existent.

Both last paragraphs would however lead into a big BC because of changes how the initialisation of the whole library would have to work.

[vanderlee]

I like the idea of supporting a standard caching mechanism, but it would have significant impact on existing users. Perhaps if we can find a way to make this work transparently by default.