Add `_observeTextReference` to update modifiers with `TextReference` types

Closes #1380

The _observeTextReference modifier tracks changed to the element associated with a TextReference type. In the content block, modifiers that use the referenced Text element can be applied. They will now properly update their contents in response to changes from the server.

In this example, the navigationTitle and badge modifiers use TextReference types, and update when the count changes.

<List style="navigationTitle(:title)">
    <Button phx-click="increment" style='badge(:badge)'>
        Increment

        <Text template="badge">
            <%= @count %>
        </Text>
    </Button>

    <Text template="title">
        <%= @count %>
    </Text>
</List>

https://github.com/liveview-native/liveview-client-swiftui/assets/13581484/73c87981-7507-42a7-921c-dfccf44e8054

liveview-native / liveview-client-swiftui

Add `_observeTextReference` to update modifiers with `TextReference` types #1383