feat(jest-config): allow to use .js files in presets (#3564)

[robin-drexler]

Summary
Allows to use a jest-preset.js instead of a jest-preset.json in presets, making it as flexible as jest.config.js and e.g. makes it possible to set config based on current environment. (#3564 (comment))
The .json file takes precedence.

Test plan
Added a unit test and tried it in a local project.
I can push that project and the necessary shell commands if necessary.
I'd like to know first, if you'd be interested at all in adding such a feature.

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[milesj]

I started working on this but haven't had time to finish it: #4499

[robin-drexler]

Ooops, totally missed your PR, sorry. I probably only searched in issues and not pull requests.

I'll close my PR in favor of yours.
Thanks for letting me know, @milesj

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