Allow optional attribute values

[Kromgart]

• Added .attr_opt() method into builder-like API.

• Allow optional attributes in view! macro, the syntax is:
  <attr-name>?=<optional_value>

• If the attribute value is None, the attribute is removed, just like a bool attribute does with a false value.

This should fix #622 (and maybe #447?)

Example:

fn main() {
    sycamore::render(|| {
        type SigOptStr<'a> = ReadSignal<Option<Cow<'a, str>>>;

        let the_class = Some("extra-dark");
        let sig_counter: Signal<i32> = create_signal(0);

        let on_btn_click = move |_| {
            sig_counter.set(sig_counter.get() + 1);
        };

        let sig_style: SigOptStr = sig_counter.map(|i| {
            if i % 2 == 1 {
                Some("color: red;".into())
            } else {
                None
            }
        });

        let sig_main_id: SigOptStr = sig_counter.map(|i| {
            if i % 2 == 0 {
                Some(i.to_string().into())
            } else {
                None
            }
        });

        let sig_data_id: SigOptStr = sig_counter.map(|i| {
            if i % 3 == 0 {
                Some(i.to_string().into())
            } else {
                None
            }
        });

        view! {
            button(on:click=on_btn_click) { "Toggle" }
            p(
                // Quoted attribute, dynamic non-option value
                "counter"=sig_counter.to_string(),
                // Plain attribute, non-dynamic option value
                class?=the_class,
                // Plain attribute, dynamic option value
                id?=sig_main_id,
                // Dashed attribute, dynamic option value
                data-id?=sig_data_id,
                // Quoted attribute, dynamic option value
                "style"?=sig_style,
            ) {
                "Some text"
            }
        }
    });
}

If this approach is acceptable - let me know, I'll add some tests into sycamore-macro (but if some other tests would be needed - let me know where).

[codecov]

Codecov Report

Attention: Patch coverage is 38.98305% with 36 lines in your changes missing coverage. Please review.

Please upload report for BASE (main@231c057). Learn more about missing BASE report.
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
packages/sycamore-web/src/node/ssr_node.rs	20.00%	12 Missing ⚠️
packages/sycamore-web/src/elements.rs	0.00%	8 Missing ⚠️
packages/sycamore-view-parser/src/parse.rs	68.42%	6 Missing ⚠️
packages/sycamore-macro/src/view.rs	63.63%	4 Missing ⚠️
packages/sycamore-reactive/src/maybe_dyn.rs	0.00%	3 Missing ⚠️
packages/sycamore-web/src/attributes.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #712   +/-   ##
=======================================
  Coverage        ?   63.50%           
=======================================
  Files           ?       52           
  Lines           ?     6823           
  Branches        ?        0           
=======================================
  Hits            ?     4333           
  Misses          ?     2490           
  Partials        ?        0           

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[lukechu10]

I was actually thinking of a bit of a different design where we can just keep the existing syntax but allow passing Option<T> as well to make it optional.

This would be done by changing the type of string attributes to something like Option<Cow<'static, str>> and automatically wrapping string values in Some(...).

We already have the machinery to do this with MaybeDyn.

[Kromgart]

Ah, ok then, no problem! :)

I was actually thinking of a bit of a different design where we can just keep the existing syntax but allow passing Option<T> as well to make it optional.

Yeah, that would look better and that was what I've tried initially, but got bogged down in untangling conflicting impls and tried this approach.