input: Write a python function to extract the text within each heading tag (<h1>, <h2>, etc.) from an HTML string.
  assert extract_headings(html_string) == ['My Portfolio', 'About Me', 'Skills', 'Experience', 'Projects']
  assert extract_headings("<h1>Title</h1><h2>Subtitle</h2>") == ['Title', 'Subtitle']
  assert extract_headings("<h1>Header1</h1><h2>Header2</h2><h3>Header3</h3>") == ['Header1', 'Header2', 'Header3']
output: def extract_headings(html_string):
      import re
      pattern = r'<h[1-6]>(.*?)<\/h[1-6]>'
      return re.findall(pattern, html_string)