generate error for unused "do" expression in EEx

[superhawk610]

This PR adds a new compilation error when EEx encounters a do expression without the <%= modifier, since this is nearly always unintentional. For example, take this block:

<% if @password %>
Temporary Password: <%= @password %>
<% else %>
Sign in with your existing password.
<% end %

At first glance, all seems well, and in the context of a full application it may take quite some time to work out that nothing is being output because you're missing the <%= opening tag. The only case I can think of where a do block is valid without the <%= opening tag is something like this:

<% if @condition do %>
  <% some_side_effect() %>
<% end %>

...but this could trivially be refactored to the (IMO) cleaner:

<% if @condition do
  some_side_effect()
end %>

Credit to @halostatue for suggesting this https://groups.google.com/g/elixir-lang-core/c/FKA9PancUm0/m/KzkT1vWuAAAJ

Closes #10536.

[fertapric]

I would also include another test that checks that it does not warn for single expressions :)

[josevalim]

@fertapric we don't need said test because otherwise the suite would be warning all the time. Generally speaking, we don't test that warnings are not emitted, because the tests would emit warnings otherwise and because it means we can't run tests concurrently.

[josevalim]

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