feat(builtins): make TextIOWrapper enumerable and readline arg optional (#331)

Resolves #131.

- Add `inherit IEnumerable<string>` to TextIOWrapper so file objects can be
  iterated line-by-line in a type-safe `for line in reader` loop without a
  cast. Maps to Python's iterator protocol; a file object is its own iterator.
- Add zero-arg `readline` / `readlines` overloads on TextIOBase so the size
  argument is optional (Python's `readline(size=-1)`), matching the existing
  two-overload pattern used by `read`.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: dbrattli

src/stdlib/Builtins.fs

@@ -12,13 +12,16 @@ type TextIOBase =
     abstract read: __size: int -> string
     abstract write: __s: string -> int
     abstract writelines: __lines: string seq -> unit
+    abstract readline: unit -> string
     abstract readline: __size: int -> string
+    abstract readlines: unit -> string list
     abstract readlines: __hint: int -> string list
     abstract tell: unit -> int
 
 type TextIOWrapper =
     inherit IDisposable
     inherit TextIOBase
+    inherit IEnumerable<string>
 
 module OpenTextMode =
     [<Literal>]

test/TestBuiltins.fs

@@ -20,6 +20,38 @@ let ``test write works`` () =
     result.Dispose()
     os.remove tempFile
 
+[<Fact>]
+let ``test TextIOWrapper can be enumerated`` () =
+    let tempFile = os.path.join (os.path.expanduser "~", ".fable_test_iter.txt")
+    let writer = builtins.``open`` (tempFile, OpenTextMode.Write)
+    writer.write "a\nb\nc\n" |> ignore
+    writer.Dispose()
+
+    let lines = ResizeArray<string>()
+    let reader = builtins.``open`` (tempFile, OpenTextMode.Read)
+
+    for line in reader do
+        lines.Add(line.Trim())
+
+    reader.Dispose()
+    os.remove tempFile
+
+    lines |> List.ofSeq |> equal [ "a"; "b"; "c" ]
+
+[<Fact>]
+let ``test readline without argument works`` () =
+    let tempFile = os.path.join (os.path.expanduser "~", ".fable_test_readline.txt")
+    let writer = builtins.``open`` (tempFile, OpenTextMode.Write)
+    writer.write "first\nsecond\n" |> ignore
+    writer.Dispose()
+
+    let reader = builtins.``open`` (tempFile, OpenTextMode.Read)
+    let line = reader.readline ()
+    reader.Dispose()
+    os.remove tempFile
+
+    line.Trim() |> equal "first"
+
 [<Fact>]
 let ``test max with two arguments works`` () =
     builtins.max (3, 5) |> equal 5