adds missing function

src/video_subtitles/convert_to_webvtt.py

@@ -0,0 +1,13 @@
+"""
+Conversion to test to webvtt format.
+"""
+
+
+import webvtt  # pylint: disable=import-error
+
+
+def convert_to_webvtt(srt_file: str, out_webvtt_file: str) -> None:
+    """Convert to webvtt format."""
+    assert srt_file.endswith(".srt")
+    assert out_webvtt_file.endswith(".vtt")
+    webvtt.from_srt(srt_file).save(out_webvtt_file)