Add 'doctest' as a cabal test

We provide a simple wrapper that executes `doctest` on our `src`
directory. This allows us to run all doctests during a regular `cabal
test` run.

hcat.cabal

@@ -50,6 +50,25 @@ executable hcat
     , hcat
   default-language: GHC2021
 
+test-suite doctest
+  type: exitcode-stdio-1.0
+  main-is: Doctest.hs
+  other-modules:
+      Paths_hcat
+  hs-source-dirs:
+      test/doctest
+  default-extensions:
+      BlockArguments
+      OverloadedStrings
+      ImportQualifiedPost
+  ghc-options: -Wall -fdefer-typed-holes
+  build-tool-depends:
+      doctest:doctest
+  build-depends:
+      base >=4.13 && <5
+    , process
+  default-language: Haskell2010
+
 test-suite spec
   type: exitcode-stdio-1.0
   main-is: Spec.hs