dev: Enable Rubocop default Layout and Style groups

In #94, I chose to disable some Rubocop groups because they weren't
related to code correctness. But this library does already conform to
most of Rubocop's default Layout and Style rules, so we can re-enable
them to keep that consistent.

.rubocop.yml

@@ -6,9 +6,15 @@ AllCops:
   # The target Ruby version must match the one in stytch.gemspec.
   TargetRubyVersion: 2.7
 
-Layout: { Enabled: false }
+Layout/LineLength: { Enabled: false }
+
 Metrics: { Enabled: false }
-Style: { Enabled: false }
+
+Style/Documentation: { Enabled: false }
+Style/For: { Enabled: false }
+Style/FrozenStringLiteralComment: { Enabled: false }
+Style/NumericPredicate: { Enabled: false }
+Style/StringConcatenation: { Enabled: false }
 
 RSpec/DescribedClass: { Enabled: false }
 RSpec/ExampleLength: { Enabled: false }