Allow casting of type variables to value variables

[shreyas1599]

Problem to Solve

Currently, instance variables can be casted into value variables and the below query is valid.

match $entity isa entity-name has attribute-one $attributeOne;
?attributeValue = $attributeOne;
fetch nestedQuery: {
    match $anotherEntity is another-entity-name, has attribute-two ?attributeValue;
    fetch $anotherEntity: attribute;
};

But the same cannot be done with a type variable. For example, the below query is invalid. The below use case is to fetch the super types of an entity and then use a subquery to fetch another entity with an attribute filter that depends on the label of the supertype. Essentially, it's trying to cast a type variable into a value variable.

Query 1

match $someEntity isa $someEntityType, has id "some id";
$someEntityType sub entity;
not { $someEntityType sub someEntityType; };
?someEntityTypeValue = $someEntityType; # illegal operation
fetch $someEntityType;
metadataOfSomeEntity: {
    match $metadata isa metadata, has entity-type ?someEntityTypeValue;
    fetch $metadata: attribute;
};

Current Workaround

To run the aforementioned query, first get the labels of $someEntityType and then pass them into another query to run the sub query from above (metadataOfSomeEntity) either as a single query by using the OR operator or running them as separate queries for each of the labels.

Proposed Solution

To be able to run Query 1 without splitting it into >1 queries - support the casting of type variables to value variables.