Add StringSyntaxAttribute to all methods and properties that take html as input

[egil]

The new StringSyntaxAttribute in .net7 signals to IDEs that a certain property/argument/field is a specific type of string, e.g. html, that allows editors to give syntax highlighting and auto completion for strings being assigned/passed to these.

We should decorate all properties and methods arguments that take HTML as input with this attribute.

For versions prior to .net7, I think we can just copy in the attribute ourselves verbatim, and it should work regardless.

Demo from Nick Chapses: https://youtu.be/Y2YOaqSAJAQ

Related: https://github.com/dotnet/runtime/issues/76138