DidYouMean::SpellChecker may not be available

[bquorning]

Fixes #7979

In #7979 it was reported that DidYouMean::SpellChecker may not always be available. We can work around that by doing a feature check before using the SpellChecker class.

I did not add any tests.

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• Wrote good commit messages.
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• Added tests.
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[bbatsov]

Thanks!