Fix ERB template highlighting

[jonatanklosko]

Currently ERB (Ruby templates) are highlighted wrongly as if they where entirely Ruby. This bug probably applies to other mixed grammars.

This was an issue in the past and was fixed in #105, but during subsequent refactoring a regression was introduced.

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[jonatanklosko]

See the original Prism highlight implementation as a reference: https://github.com/PrismJS/prism/blob/v1.30.0/prism.js#L661-L670.

Calling before-tokenize above modifies prismConfig.code. For example, given this snippet:

<div class="flex gap-2">
  <%= link_to "Controller error", page_controller_error_path,
      class: "btn btn-error btn-soft" %>
  <%= link_to "View error", page_view_error_path,
      class: "btn btn-error btn-soft" %>
  <%= link_to "View sytnax error", page_view_syntax_error_path,
      class: "btn btn-error btn-soft" %>
  <%= link_to "Missing view error", page_missing_view_error_path,
      class: "btn btn-error btn-soft" %>
  <%= link_to "Route error", "/page/unknown-route",
      class: "btn btn-error btn-soft" %>
</div>

before-tokenize rewrites the code to:

<div class="flex gap-2">
  ___ERB0___
  ___ERB1___
  ___ERB2___
  ___ERB3___
  ___ERB4___
</div>

it is expected that we pass the modified code to prism.tokenize, rather than the original code, as currently done.