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.
