This commit extends the `ActionView::Helpers#translate` (and by way of
alias, `#t`) helper methods to accept blocks.

When invoked with a block, the `translate` call will yield the
translated text as its first block argument, along with the resolved
translation key as its second:

```erb
<%= translate(".key") do |translation, resolved_key| %>
  <span data-i18n-key="<%= resolved_key %>"><%= translation %></span>
<% end %>
```

In cases where relative translation keys are foregone in lieu of fully
qualified keys, or if the caller is not interested in the resolved key,
the second block argument can be omitted:

```erb
<%= translate("action.template.key") do |translation| %>
  <p><%= translation %></p>
  <p><%= translation %>, but a second time</p>
<% end %>
```

A benefit of yielding the translation is that it enabled template-local
variable re-use. Alternatively, [`Object#tap`][tap] could be used.

Prior to this commit, however, the resolution of the translation key was
internal to `ActionView`, and unavailable to the caller (unless they
were willing to explicitly determine the resolved key themselves). By
making it available as a block parameter, it could be used to annotate
the translated value in the resulting elements.

[tap]: https://ruby-doc.org/core-2.7.0/Object.html#method-i-tap