Support each_with_index and times in instance templates

[sbsoftware-agent]

Summary

• Extend template iterator block handling to support each_with_index and times (receiver + block), alongside existing each.
• Ensure iterator block bodies are evaluated through the template evaluator so nested tag calls, string literals, control flow, and to_html objects render consistently.
• Preserve block argument behavior, including omitted args for times blocks.

Changes

• Updated src/instance_template.cr in ToHtml.to_html_eval_exp:
• Added allowlisted handling for each_with_index blocks.
• Added allowlisted handling for times blocks.
• Kept each aligned with the same block-evaluation behavior and made iterator pretty-print separation independent of injected index args (so omitted block args are safe).
• Added spec/instance_template/iterator_blocks_spec.cr:
• each_with_index example with nested tags, literals, and if/unless.
• times examples covering both do |i| and do ... end forms, including nested control flow.

Testing

• Ran: crystal spec (using CRYSTAL_CACHE_DIR=/tmp/crystal-cache-html because default cache path is not writable in this environment)
• Result: 34 examples, 0 failures