Add block prop for ButtonGroup for full width

[tqwewe]

In a particular use case of mine, I'd like the ButtonGroup component to stretch to full width, with each button child being equal width filling the space.

For example, something similar to:

<div class="flex w-full"> <!-- ButtonGroup -->
    <button class="w-1/3">One</button> <!-- Button -->
    <button class="w-1/3">Two</button> <!-- Button -->
    <button class="w-1/3">Three</button> <!-- Button -->
</div>

From:

view! {
    <ButtonGroup block=true>
        <Button>"One"</Button>
        <Button>"Two"</Button>
        <Button>"Three"</Button>
    </ButtonGroup>
}

Additionally, the ButtonGroup component is missing class and style props, so at this stage I don't even have a way of manually putting class="w-full" on the ButtonGroup sadly.

[luoxiaozero]

I'll add the class and block attributes later, you can temporarily fix them now with the following method:

use leptos_meta::Style;

view! {
    <div class="my-button">
        <Style>
            ".my-button > .thaw-button-group {
            }
           .my-button > .thaw-button {
            }"
        </Style>
        <ButtonGroup block=true>
            <Button>"One"</Button>
            <Button>"Two"</Button>
            <Button>"Three"</Button>
        </ButtonGroup>
    </div>
}