Fix HEREDOC parsing inside BEGIN blocks (#366)

[fglock]

Summary

Fixes #366 - HEREDOC parsing now works correctly inside BEGIN blocks.

When a HEREDOC is declared inside a BEGIN block (e.g., BEGIN { eval <<'END' }), the heredoc content appears on subsequent lines after the block closes. However, BEGIN blocks execute immediately during parsing, before the newline that triggers heredoc content processing.

Changes

• Parser.java: Added heredocSkipToIndex field to track where to skip after pre-processed heredocs
• SpecialBlockParser.java: Before executing BEGIN blocks, process any pending heredocs by:
  1. Finding the next NEWLINE token
  2. Temporarily advancing to process heredocs
  3. Storing the post-heredoc position
  4. Restoring tokenIndex for normal parsing
• Whitespace.java: When encountering a NEWLINE with heredocSkipToIndex set, skip past already-processed heredoc content

Test Plan

• Minimal reproducer now works:

  BEGIN {if (0){1}else {eval <<'END'}}print "after\n";
  hello
  END

• CPAN::Meta::YAML pattern works (simplified test case)
• Multiple heredocs in BEGIN block work
• All unit tests pass
• Output matches system Perl for all test cases

Impact

This enables cpanm and other tools that load CPAN::Meta::YAML, which uses this pattern for optional dependency loading.

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