Preserve new lines in string portion of interpolated string (#1452)

* Preserve new lines in string portion of interpolated string

New lines were not being preserved when combining the string tokens of an interpolated string expression. If consecutive tokens are on different lines, then we add an additional NewLine in between.

Fixes #1451

* Minor tweak

Co-authored-by: nojaf <florian.verdonck@outlook.com>

src/Fantomas.Tests/InterpolatedStringTests.fs

@@ -146,3 +146,20 @@ $"\"{bar}\" {1} {2}"
         """
 $"\"{bar}\" {1} {2}"
 """
+
+[<Test>]
+let ``multiline string literal, issue 1451`` () =
+    formatSourceString
+        false
+        "
+$\"\"\"one: {1}<
+>two: {2}\"\"\"
+"
+        config
+    |> prepend newline
+    |> should
+        equal
+        "
+$\"\"\"one: {1}<
+>two: {2}\"\"\"
+"

src/Fantomas/TokenParser.fs

@@ -579,6 +579,17 @@ let private identIsDecompiledOperator (token: Token) =
     token.TokenInfo.TokenName = "IDENT"
     && decompiledName <> token.Content
 
+let private extractContentPreservingNewLines (tokens: Token list) =
+    let rec loop result =
+        function
+        | [] -> result
+        | [ final ] -> final.Content :: result
+        | current :: ((next :: _) as rest) when (current.LineNumber <> next.LineNumber) ->
+            loop ("\n" :: current.Content :: result) rest
+        | current :: rest -> loop (current.Content :: result) rest
+
+    loop [] tokens |> List.rev
+
 let ``only whitespaces were found in the remainder of the line`` lineNumber tokens =
     tokens
     |> List.exists
@@ -732,7 +743,7 @@ let rec private getTriviaFromTokensThemSelves
 
     | EndOfInterpolatedString (stringTokens, interpStringEnd, rest) ->
         let stringContent =
-            [ yield! (List.map (fun t -> t.Content) stringTokens)
+            [ yield! extractContentPreservingNewLines stringTokens
               yield interpStringEnd.Content ]
             |> String.concat String.Empty