fix: handle heredocs with trailing code

[jeanpaulsio]

Summary

• Heredocs with trailing code on the opener line now parse correctly: foo(<<~SQL), <<~HEREDOC.strip
• Uses a hybrid approach: full token when no trailing code (preserves body highlighting), opener-only token when trailing code exists (lets the parser handle ), .method, etc.)
• All 105 existing tests pass, no regressions

What changed

Two files modified (src/tokens.ts, src/syntax.grammar):

• tryMatchHeredoc now detects trailing code after the opener and returns either the full heredoc length or just the opener length
• Grammar comment updated to document the hybrid approach

Test plan

• All 105 existing tests pass
• foo(<<~SQL)\n SELECT 1\nSQL — parses as MethodCall with Heredoc arg, no error nodes
• <<~HEREDOC.strip\n hello\nHEREDOC — parses as MethodCall chain, no error nodes
• a << b — still parsed as left shift, not heredoc
• def foo\n x = <<~SQL\n SELECT 1\n SQL\n x\nend — heredoc inside method body works