feat: add option to add `__typename` to interfaces

[gabrielschulhof]

The new option addTypenameToInterfaces adds the __typename field to types resulting from interfaces. The field admits a union of string values, each the name of one of the concrete types implementing the interface.

Also adds field typeUnionOperator to class BaseVisitor so we don't have the literal ' | ' in a bunch of places.

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Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.

This will allow us to treat incoming objects uniformly using their parent type, and will also allow us to type cast appropriately when the handling of the individual object begins to diverge based on their type.

Related #10522

Type of change

Please delete options that are not relevant.

• [x] New feature (non-breaking change which adds functionality)
• [x] This change requires a documentation update

Screenshots/Sandbox (if appropriate/relevant):

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How Has This Been Tested?

A unit test is included. The remaining unit tests verify that no change occurs when the option is absent.

Test Environment:

• OS: OSX
• NodeJS: 20

Checklist:

• [x] I have followed the CONTRIBUTING doc and the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes

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