Merge adjacent StringLiterals before yielding

[Blacksmoke16]

Fixes #16413

Joins adjacent StringLiteral values before yielding within interpolations to ensure string is properly escaped if needed. E.g. in the following example:

{% begin %}
  {% verbatim do %}
    {%
      puts %(#{2} ##{"{"}}) # => 2 #{}
    %}
  {% end %}
{% end %}

The string value " \#{}" is yielded all at once, versus before each character ( , #, {, and }) were all yielded on their own which prevented proper escaping of the interpolation.