case insensitive for HTML attributes

[RobinDev]

The HTML parser treated attribute names in a case-sensitive manner, which is not compliant with the HTML specifications.

This behavior caused incorrect escaping of tags containing attributes with uppercase letters, such as the viewBox attribute in SVG tags.

This change updates the PARTIAL_ATTRIBUTENAME regular expression to accept both uppercase and lowercase letters, ensuring that HTML attributes are correctly recognized regardless of their case.

[colinodell]

Thanks for proposing this change! Would you mind sharing a specific example of the input you're using, and the incorrectly-escaped output? I'd like to include that as a regression test, and also use it to compare how other CommonMark-compliant parsers are handling this.