Added new function isEelExpression
Added new function isEelExpression

The function was added to provide a function for validity checks without any need to duplicate the code which was previously used in the first if-condition of the function evaluateEelExpression.

As @kitsunet mentioned in https://github.com/neos/eel/pull/2 the variable `matches` is not necessary, but I've used it to avoid calling `preg_match(Package::EelExpressionRecognizer, $expression, $matches)` inside the existing function `evaluateEelExpression` and `preg_match(Package::EelExpressionRecognizer, $expression)` inside the new function `isEelExpression`, because to me this seems like duplicated code.
I'm happy with both solutions, so feel free to change it if you decide to remove the variable.
dlubitz/flow-development-collection,neos/flow-development-collection,fcool/flow-development-collection,kdambekalns/flow-development-collection,albe/flow-development-collection,kdambekalns/flow-development-collection,daniellienert/flow-development-collection,robertlemke/flow-development-collection,dlubitz/flow-development-collection,robertlemke/flow-development-collection,fcool/flow-development-collection,robertlemke/flow-development-collection,fcool/flow-development-collection,dlubitz/flow-development-collection,neos/flow-development-collection,albe/flow-development-collection,neos/flow-development-collection,neos/flow-development-collection,kdambekalns/flow-development-collection,kdambekalns/flow-development-collection,daniellienert/flow-development-collection,albe/flow-development-collection,daniellienert/flow-development-collection,bwaidelich/flow-development-collection,fcool/flow-development-collection,bwaidelich/flow-development-collection,robertlemke/flow-development-collection,bwaidelich/flow-development-collection