Add a new prededined function fileExists

The `fileExists` function will return true if the file exists, false if it doesn't.
The parameter is either a `java.lang.String`, a `java.io.File` or a `java.nio.file.Path`.
Add a new prededined function fileExists

The `fileExists` function will return true if the file exists, false if it doesn't.
The parameter is either a `java.lang.String`, a `java.io.File` or a `java.nio.file.Path`.