[typescript-fetch] Add stringEnums option

[mkusaka]

This PR adds a stringEnums option to typescript-fetch generator, similar to typescript-axios (#11368).

Closes #11480

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[mkusaka]

Oh. npm run build for typescript-fetch/es6-target sample fail due to low typescript version(as const assertion introduced typescript 3.4, but this sample uses typescript ^2.4).

Other es6-target samples such as typescript-rxjs and typescript-axios use typescript 3.6 or above. So, I think typescript-fetch/es6-target can use typescript 3.6 or above.

EDIT:
Because of default supported version may be 2.x, this option should respect typescript version.
I think it is sufficient to respect the typescriptThreePlus option for this time.

[macjohnny]

LGTM

[macjohnny]

thanks for your contribution