Move code to Internal module and add tests

package.yaml

@@ -4,7 +4,7 @@ license: MIT
 author: "Alexander Kobjolke"
 maintainer: "alex@jakalx.net"
 copyright: "Alexander Kobjolke 2023"
-description: "Hcat example from the book 'Effective Haskell'"
+description: "HCat example from the book 'Effective Haskell'"
 
 extra-source-files:
   - README.org
@@ -20,6 +20,12 @@ dependencies:
 ghc-options:
   - -Wall
   - -Wunused-packages
+  - -fdefer-typed-holes
+
+default-extensions:
+  - GHC2021
+  - BlockArguments
+  - OverloadedStrings
 
 library:
   source-dirs: src
@@ -34,3 +40,17 @@ executables:
       - hcat
     verbatim:
       default-language: GHC2021
+
+tests:
+  spec:
+    cpp-options: -DTEST
+    main: Spec.hs
+    source-dirs:
+      - test
+    dependencies:
+      - hcat
+      - hspec
+      - QuickCheck
+      - quickcheck-instances
+      - quickcheck-text
+    build-tools: hspec-discover