Make the ? suffix for empty attributes optional

[lambda-fairy]

Summary

Maud no longer requires ? after an empty attribute. For example, this code

html! {
    input type="checkbox" checked? disabled?[is_disabled];
}

can now be written as

html! {
    input type="checkbox" checked disabled[is_disabled];
}

The old syntax is still allowed, but soft-deprecated. It will be removed in some future release (TBD).

Rationale

The ? was necessary back when braces were optional, such that a bare identifier (e.g. title) could mean either an attribute or an element.

But with #137 making braces mandatory, there's no need for the ? anymore – it is perfectly unambiguous without it.

[lambda-fairy]

TODO:

• Update changelog
• Update docs