feat: add `text/plain` fallback response

[Phillip9587]

Based on the work in #56.

Adds a fallback text/plain response when the client does not accept html.

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Package	New capabilities	Transitives	Size	Publisher
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[Phillip9587]

@wesleytodd We maybe should use negotiator not accepts because negotiator is more lightweight compared to accepts and we don't need the mime types support from accepts.

[wesleytodd]

Yeah, probably true. Frankly I dislike all the content type negotiation packages we have, they could use some consolidation and cleanup to make it more clear when to use which.

[Phillip9587]

I don’t think we can include this in a 2.x release since it introduces a breaking change. Currently, finalhandler always returns a text/html response, and even though changing this might be more correct, it could still potentially break existing users relying on the current behavior.