When translating a `<button>` element's contents, it is tedious to make
the translation text available to a block scope.
For instance, when rendering a `<button type="submit">` with an SVG
element as its child, passing translated label text to that SVG
element's [`<title>`][svg-title] element requires an extra call to
`I18n.translate`.
Prior to this commit, doing so would require a double lookup of the
translation key:
```erb
<%# one time here, implicitly %>
<%= form.button do %>
<svg>
<title>
<!-- one time here, explicitly -->
<%= translate("helpers.submit.post.create") %>
</title>
<!-- ... -->
</svg>
<% end %>
```
This commit modifies the `ActionView::Helpers::FormBuilder#button` to
check for invocations that are passed a block, and conditionally yield
the contents of `submit_default_value` as the argument.
The new view code might look something like this:
```erb
<%= form.button do |text| %>
<svg>
<title><%= text %></title>
<!-- ... -->
</svg>
<% end %>
```
Callers of the helper are still free to omit the block parameter.
[svg-title]: https://developer.mozilla.org/en-US/docs/Web/SVG/Element/title
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