Implement multiple optional attributes

[ealmloff]

Introduces a new syntax for creating and combining optional class and style attributes:

div {
    class: if true { "m-2" },
    class: if false { "p-2" },
}

This is breaking because we previously accepted:

div {
    class: "m-2",
    class: "p-2",
}

and rendered it as

div {
    class: "p-2",
}

It now renders as:

div {
    class: "m-2 p-2",
}

Closes #1246

[kaivol]

Maybe I'm just bad at googling or documentation reading, but I didn't find this feature until I came across this PR.
I would have expected it to be mentioned here.

[ealmloff]

Maybe I'm just bad at googling or documentation reading, but I didn't find this feature until I came across this PR. I would have expected it to be mentioned here.

Documentation for this feature is added in DioxusLabs/docsite@f63e9e0