Add a warning if timber plugin or class deosn't exist

[palmiak]

Now if user will install Timber ACF WP Blocks he'll get a fatal error if Timber is not installed. It should be fixed.

[landwire]

Could that not be solved by adding "timber" as a required package in the composer.json?

"require": {
    "php": ">=5.6",
    "timber/timber": "1.*"
}

In the end you need timber for this to work, right?

[jdamner]

Timber can be installed in other methods/contexts, so would be better to check if (class_exists(“Timber”))