Add label_style modifier

[carson-katri]

Closes #478

This sets up a solution for creating custom styles purely on the Elixir side.

The custom elements <icon /> and <title /> can be used within the <label_style:custom_style>, and provide access to the content of the label they represent. Then this style can quickly be applied to all labels within the <VStack>.

<VStack modifiers={label_style(@native, style: :custom_style)}>
    <Label system-image="hand.raised.fill">Greet</Label>
    <Label system-image="eraser.line.dashed.fill">Erase</Label>
    <Label system-image="lasso.and.sparkles">Wrangle</Label>

    <%!-- Create the custom style, which is applied to all 3 labels --%>

    <label_style:custom_style>
        <Grid modifiers={padding(@native, all: 8)}>
            <GridRow>
                <Text>Icon</Text>
                <Text>Title</Text>
            </GridRow>
            <GridRow modifiers={foreground_style(@native, primary: {:color, :green})}>
                <%!-- These elements are only valid within this style block. --%>
                <icon />
                <title />
            </GridRow>
        </Grid>
    </label_style:custom_style>
</VStack>

[bcardarella]

can you change label_style to label-style instead? We want to conform to hyphenation within the markup

[bcardarella]

oh it is linking back to the label_style modifier... let's discuss this on standup. Having underscored elements is going to throw people off