Add type to all nodes

[RubenVerborgh]

Suggestion by @AlexeyMz at https://github.com/RubenVerborgh/SPARQL.js/pull/80#issuecomment-516328846:

May I also suggest to add type constants to other node types as well?

Here are the current typings: https://github.com/DefinitelyTyped/DefinitelyTyped/blob/d2d3a685fe949a4e938fe2913597e49d1e703ef5/types/sparqljs/index.d.ts

It would be great to make these changes:

1. add type equal to grouping for Grouping, ordering for Ordering, variable for VariableExpression;
2. change updateType property to type in InsertDeleteOperation (currently half operations use updateType and the other one just type).