feat(ts): add flattenSubclassesToUnion param

[mttrbrts]

Some client applications have a preference for union types over inheritance and subclasses (even inside TypeScript).

This change introduces a new parameter that lets the client application control which variant to use when generating TypeScript from Concerto.

Changes

• Adds flattenSubclassesToUnion parameter to the TypeScriptVisitor. The existing behaviour is still the default

For example, when flattenSubclassesToUnion = true, we generate the following code

export interface IDecorator extends IConcept {
   name: string;
   arguments?: DecoratorLiteralUnion[];
   location?: IRange;
}

instead of the default

export interface IDecorator extends IConcept {
   name: string;
   arguments?: IDecoratorLiteral[];
   location?: IRange;
}

Author Checklist

• Ensure you provide a DCO sign-off for your commits using the --signoff option of git commit.
• Vital features and changes captured in unit and/or integration tests
• Commits messages follow AP format
• Extend the documentation, if necessary
• Merging to main from fork:branchname

[coveralls]

coverage: 98.886% (-0.02%) from 98.91%
when pulling 97640fc on mr-flattensubclasses
into e38396e on main.

[dselman]

Nice.