[typescript-fetch] fix snakecase discriminator

[ctjhoa]

Fix #7080

Discriminator should be based on propertyBaseName instead of propertyName like in the following languages: csharp, python, java

I've added reproduction config based on modules/openapi-generator/src/test/resources/3_0/petstore-with-fake-endpoints-models-for-testing.yaml

cc @TiFu (2017/07) @taxpon (2017/07) @sebastianhaas (2017/07) @kenisteward (2017/07) @Vrolijkx (2017/09) @macjohnny (2018/01) @topce (2018/10) @akehir (2019/07) @petejohansonxo (2019/11) @amakhrov (2020/02) @davidgamero (2022/03) @mkusaka (2022/04) @eriktim @stephanpelikan

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[TiFu]

LGTM!