Parser: Convert literals to static text when transforming postfix/ternaries with `transform_conditionals`

[marcoroth]

A future optimization could be to extract literal values directly as LiteralNodes or the proper equivalent. So that:

<%= condition ? 'yes' : 'no' %>

could become:

- <% if condition %><%= 'yes' %><% else %><%= 'no' %><% end %>
+ <% if condition %>yes<% else %>no<% end %>