Allow disjoint types in type labels

[flyingsilverfin]

Problem to Solve

We can easily implement an extension to TypeQL - a union/disjunction of types, as an alternative to a single type label. This would make a single TypeQL query expressive without having to resort to disjunctions or separate queries.

Current Workaround

To query for instances of two types, we must do for example:
match $x isa $t; { $t type person;} or { $t type house;};
or run two separate queries.

Proposed Solution

We could instead write match $x isa person|house;, which allows lumping two possibly disjoint types into the Label for the isa.

This should work for anything in the query that contains a Label constraint, and can be directly implemented by extending the grammar and the parsed Constraint to support a set of label strings.

[dmitrii-ubskii]

This may be a bigger change than was initially thought.

Because the type constructs are shared between different kinds of queries, viz. match and define, allowing a label to be a union of labels across the board would lead to define foo sub bar|baz, owns name|age; being a valid query from the grammar's point of view. It wouldn't make a whole lot of sense to just restrict the X|Y construct to the isa constraint either.

This can be mitigated by splitting off the definition rules off from type constraints, but that would be its own undertaking.