fix: Fix spreading abstract fragments into abstract selections widening the result type

[kitten]

Resolves #100

Summary

This improves the type inference for two cases specifically.

When spreading a fragment of an interface (potentially itself) into an interface selection, the type wasn't narrowed even if we had the fragment available and were doing a submatch. This lead to poor type matches when a user wasn't specifying __typename above and below the spread although this isn't necessary, since we have all necessary type information.
We can generically solve this by keeping the PossibleType narrow, specifically by skipping getSelection and using getPossibleTypeSelectionRec directly.

Specifically, when using an unmasked fragment document and repeating the above a similar issue would occur. In such a case the type checker would be forced to accept two unions of two types that aren't mergeable.

There isn't a great solution for this without refactoring a whole lot and making output types less readable, while hurting performance in the process.
There is however a simple solution.

We now always attach a __typename?: PossibleType optional property to possible type selections. This attaches a type to match against for the type checker to merge all possible types’ selections.
While not the perfect solution, this shouldn't have any negative consequences.

• The property is optional, so unlikely to be freely used without being requested
• The property may explicitly become never when a branch is missing a typename providing a visual indicator that __typename isn't selected in all cases
• The property is already likely to be requested, and even if it's aliased, this should work correctly
• __typename is unlikely to be aliased (which we can also lint for), since it's a system field and unique in behaviour

Set of changes

• Skip getSelection and use getPossibleTypeSelectionRec directly, where possible
  • Provide the narrowed PossibleType in nested fragment selections
• Add a __typename?: PossibleType property to possible type selections
• Add tests for these cases

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