Disambiguate if expressions in rsx by requiring curlies, allow shorthand component/element initialization

[jkelleyrtp]

This PR changes how rsx interprets expressions, requiring them to be wrapped in curly braces.

This is a hugely breaking PR but allows a number of goodies like letting us keep consistency of if/for special cases and shorthand struct initialization.

It could also lead the way to improved autocomplete via partial expansion, however this is not yet implemented.

In it's current form it provides way better autocomplete when working inside curly braces and better autocomplete of attributes and props.

So, this works now:

if cond {
    div {}
} else {
   h1 {}
}

And so does this:

let a = 123;
let b = 456;
let c = 789;
let children = render!{ "hi" };

rsx! {
    Component { a, b, c, children }
}

This also supports elements:

let class = "123";

rsx! {
    div { class } 
}

However we don't special-case children and currently panic if an attribute named "children" is present.

In theory we could accept the children argument in elements, but I just didn't get around to it here.

[ealmloff]

I think allowing you to do normal rsx elements in match arms would be more consistant with if statements and loops in the future, but it could be in a different PR

[ealmloff]

Looks good! Excited for better autocomplete