Preserve the last empty comment line after eval plugin (#631)

```haskell
-- >>> foo
--
-- >>> bar
```

After running the plugin on both prompts we end up with this:

```haskell
-- >>> foo
-- foo_output
--
-- >>> bar
-- bar_output
```

instead of

```haskell
-- >>> foo
-- foo_output
-- >>> bar
-- bar_output
```

Author: expipiplus1

plugins/default/src/Ide/Plugin/Eval.hs

@@ -117,7 +117,14 @@ extractMatches = goSearch 0 . maybe [] T.lines
         r = Range p p'
         p = Position line 0
         p' = Position (line + spliceLength) 0
-        spliceLength = length (takeWhile looksLikeSplice (l : ll))
+        spliceLines = takeWhile looksLikeSplice (l : ll)
+        -- Don't include the last line if it's an empty comment.
+        -- Do this to preserve spacing between consecutive splices
+        isEmptyComment = (== "--")
+        spliceLength = case spliceLines of
+                         [] -> 0
+                         ls | isEmptyComment (last ls) -> length ls - 1
+                            | otherwise -> length ls
 
 provider :: CodeLensProvider
 provider lsp _state plId CodeLensParams {_textDocument} = response $ do

test/functional/Eval.hs

@@ -91,6 +91,8 @@ tests = testGroup
   $ goldenTest "T25.hs"
   , testCase "local imports"
   $ goldenTest "T26.hs"
+  , testCase "Preserves one empty comment line after prompt"
+  $ goldenTest "T27.hs"
   ]
 
 goldenTest :: FilePath -> IO ()

test/testdata/eval/T27.hs

@@ -0,0 +1,4 @@
+module T27 where
+
+-- >>> ()
+--

test/testdata/eval/T27.hs.expected

@@ -0,0 +1,5 @@
+module T27 where
+
+-- >>> ()
+-- ()
+--