When translating a `<label>` element's contents, it is difficult (or
"possible", yet undocumented) to make the translation text available to
a block scope.

For instance, when rendering a `rich_text_area`, passing the
`aria-label` attribute might be important.

Prior to this commit, doing so would require a double lookup of the
translation key:

```erb
<%# one time here, implicitly %>
<%= form.label(:content) do %>
  <%= form.rich_text_area(
    :content,
    # one time here, explicitly
    "aria-label" => translate("helpers.label.post.content"),
  ) %>
<% end %>
```

The current implementation of the `#label` helper method already yields
an instance of `ActionView::Helpers::Tags::Label::LabelBuilder`, but
that class is undocumented. Instance of that class respond to
`#translation` calls, which will return the translated text content.

By aliasing `#translation` to `#to_s`, we're able to expose that value
without the burden of exposing an additional class to the public API.
Instead, view-level interpolation (either `<%= %>`, `#{ }`, or direct
calls to [`capture`][capture] will coerce the value to a String, and
implicitly invoke `#translation`.

The new view code might look something like this:

```erb
<%= form.label(:content) do |label| %>
  <%= form.rich_text_area(:content, "aria-label" => label) %>
<% end %>
```

Callers of the helper are still free to omit the block parameter.

[capture]: https://api.rubyonrails.org/classes/ActionView/Helpers/CaptureHelper.html#method-i-capture