This PR makes it so that adding a feature naming description when there
is no pattern is disallowed. It also changes the validation for feature
naming slightly so that it can return multiple errors at once.
This PR updates the back-end handling of feature naming patterns to add
implicit leading `^`s and trailing `$`s to the regexes when comparing
them.
It also adds tests for the new behavior, both for new flag names and for
examples.
## Discussion points
Regarding stripping incoming ^ and $: We don't actually need to strip
incoming `^`s and `$`s: it appears that `^^^^^x$$$$$` is just as valid
as `^x$`. As such, we can leave that in. However, if we think it's
better to strip, we can do that too.
Second, I'm considering moving the flag naming validation into a
dedicated module to encapsulate everything a little better. Not sure if
this is the time or where it would live, but open to hearing
suggestions.
