feat(html): add `normalize` function for HTML entities (#4523)

[lionel-rowe]

Fixes #4523

Various bikeshedding things:

• Given its dual use for XML, should the top-level dir have its name changed from html? If so, to what?
  • xml suffers the same issue but in reverse
  • html_and_xml is pretty gross
  • markup seems overly vague
  • sgml_like might be technically correct, but would the average web developer think to look there?
• Is it worth exporting escapeAllCharsAsHex? Could be useful for users wanting finer-grained control than the 2 normalization forms, but it's a very simple function to replicate
• Is the name NormalizationForm OK or is it too confusing with Unicode normalization forms (NFC, NFD, etc?)