lib/helpers: first shellcheck pass

Quote things, SC2268, SC2143, SC2181, SC2162, SC2016, SC2013, &c.

Rewrite globbing per `shellcheck`’s SC2013, and alsö s/typeset/local/g. Eliminate `compgen` where possible.

Alsö: use the existing utility functions `_bash-it-get-component-type-from-path` and `_bash-it-get-component-name-from-path`, which just use parameter substitution anyway. Why was `sed` here?

Alsö, don't add not-existing directories to `$PATH` in `pathmunge()`.

Finally, merge PR #1865 from NoahGorny...and clean it a bit...

test/plugins/ruby.plugin.bats

@@ -6,6 +6,8 @@ load ../test_helper_libs
 function local_setup {
   setup_test_fixture
 
+  _command_exists "ruby" && mkdir -p "$(ruby -e 'print Gem.user_dir')/bin"
+
   export OLD_PATH="$PATH"
   export PATH="/usr/bin:/bin:/usr/sbin"
 }