Validate format for dateFormat and addToDate method

**Is your feature request related to a problem? Please describe.**

`dateFormat` and `addToDate` function does not validate the dateFormat supplied as input to the function and accepts any String.

**Describe the solution you'd like**

Restrict access to only valid dateFormats supported by [SimpleDateFormat](https://docs.oracle.com/javase/7/docs/api/java/text/SimpleDateFormat.html). The validation should be compile-time rather than at runtime.

**Additional context**

See [Xtext Validations](https://www.eclipse.org/Xtext/documentation/303_runtime_concepts.html#validation) for pointers. You will either need to change the grammar or add custom validation.