We don't refer to "readline functions" anywhere else, and "injecting"
them "into the reader" is an overly jargony way of expressing it that
only makes sense to someone familiar with the internals. And even then
the term "readline" is already taken by the "readline" library, used
by bash et al, but not by us.
So we pick the term "input functions", like we did in bind.
See https://stackoverflow.com/questions/55542839/what-does-commandline-f-repaint-in-fish-shell/55543411#55543411.
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