stdio: add readfln and File.readfln

[pbackus]

Compared to readf, these functions provide a less error-prone way to read a single line of formatted input.

Fixes #10370

[dlang-bot]

Thanks for your pull request and interest in making D better, @pbackus! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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Testing this PR locally

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dub run digger -- build "master + phobos#10656"