input: Write a function to convert a given HTML string to plain text without using any libraries. If the function encounters an HTML tag, it should skip the content inside that tag, and only append characters outside of tags to the output string. The function should return the resulting plain text string.
  
  assert html_to_text("<p>Hello, <b>world</b>!</p>")=="Hello, world!"
  assert html_to_text("<h1>Heading</h1><p>Paragraph</p>")=="HeadingParagraph"
  assert html_to_text("<a href='url'>Link</a>")=="Link"
  assert html_to_text("<div>Content</div>")=="Content"
  assert html_to_text("")==""
output: def html_to_text(html):
      result = []
      inside_tag = False
      for char in html:
          if char == '<':
              inside_tag = True
          elif char == '>':
              inside_tag = False
          elif not inside_tag:
              result.append(char)
      return ''.join(result)