This removes one more C dependency (see #4535).
yamlet also performs better than yaml (though YAML parsing is not
a bottleneck).
[API change] Text.Pandoc.Shared: export `extractYaml`.
[Behavior change] yamlet is YAML 1.2, whereas yaml was YAML 1.1.
This means that a field containing `no` or `yes` will now parse as a
string rather than a Boolean.
Closes#8759.
John MacFarlane
ac1d88b0ae
