[typescript-fetch] support "x-enum-descriptions"

[RyosukeFukushima]

refs #1693

x-enum-descriptions was not supported in typescript-axios but not in typescript-fetch.

Similar pull requests:

• [typescript-axios] support "x-enum-descriptions" #7477
• [JavaSpring] Add Javadoc to enum (x-enum-descriptions) #14123

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[joscha]

was not supported in typescript-axios

->

was not supported in typescript-axios

?

@RyosukeFukushima would it make sense to change one of the yaml spec files to add an enum description somewhere, so one of the generated samples shows the change?

cc @macjohnny

[macjohnny]

thanks for your contribution!