Use Validator.nu as default parser when TagSoup is unavailable

[GuntherRademacher]

This change will use Validator.nu as the default HTML parser, when TagSoup is unavailable, or fall back to XML parsing, if Validator.nu is unavailable, too.

When HTMLPARSER option method is used to override the default, then an error message is produced, if the specified parser is unavailable.

[GuntherRademacher]

The documentation has been updated to explain this behavior:

• https://docs.basex.org/12/Parsers
• https://docs.basex.org/12/HTML_Functions
• https://docs.basex.org/12/Options