feat: allow string responses

[sr258]

I've added the support for returning a string value in a response instead of only allowing JSON. Adding the change to the Storybook docs in packages/mock-addon-docs requires some bigger changes in the test files, so I haven't done that yet because I wanted to wait for your feedback on whether you want to accept the PR and if so, how it should be added to the docs.

Closes #200 .

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[Mattsi-Jansky]

I would love this feature. It is quite a severe limitation to only be able to return objects and arrays. Hope the PR gets merged