[typescript-fetch] Update instanceOf function to check baseName

[jasperpatterson]

The instanceOf type guard functions check for TypeScript property names (camelCase) instead of JSON property names (in my case snake_case), causing runtime type validation to fail for discriminated unions.

I believe the template should use {{baseName}} (JSON property) instead of {{name}} (TypeScript property)?

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[wing328]

@jasperpatterson thanks for the PR

what if the baseName contains special characters (e.g. modified-date), would it still work?

in other client generators, there are mappings between object property name (name) and JSON property name (baseName). Not sure whether typescript-fetch (or TypeScript in particular) also uses mapping somehow

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