fix(lib/shopt): turn on "extglob" by default

We thought that we already have `extglob` turned on, but it was just
turned on by bash-completion that we loaded from "lib/bourne-shell".
When the sytem does not have bash-completion, the extglob settings are
not enabled and thus causes an issue with constructs using extglob.
We already turn on many glob features in "lib/shopt", and there does
not seem to be reason not to turn on "extglob".  In this patch, we
enable it by default.

lib/shopt.sh

@@ -18,6 +18,9 @@ PROMPT_DIRTRIM=${PROMPT_DIRTRIM:-2}
 # E.g. typing !!<space> will replace the !! with your last command
 bind Space:magic-space
 
+# Turn on extended glob patterns such as @(...), *(...), ?(...), and +(...)
+shopt -s extglob
+
 # Turn on recursive globbing (enables ** to recurse all directories)
 shopt -s globstar 2> /dev/null