Parser: Allow inline in expressions outside of case statements

[ttilberg]

Currently using value in pattern without an accompanied case statement raises ERBControlFlowScopeError.

<% if value in pattern %>
  True
<% end %>

`<% in %>` appears outside its control flow block. Keep ERB control flow statements together within the same HTML scope (tag, attribute, or content). (Herb Parser [ERB_CONTROL_FLOW_SCOPE_ERROR])

Now a pattern match predicate only increments in_node_count when it's accompanied by a case statement.

I used Claude to help me understand how to make this change -- so please look it over and determine if this is the correct type of fix.

Fixes #1758

[marcoroth]

Awesome catch, thank you so much @ttilberg! 🙏🏼