Add fragment composition support for JS::File

[sbsoftware-agent]

Summary

Adds fragment composition to JS::File so multiple independent JS contributions can be appended into a single file output in deterministic order, including reopen-style class usage.

Changes

• Added a new js_fragment macro on JS::File for explicit fragment contribution.
• Refactored JS::File emission to:
• keep js_class and js_function output,
• append registered fragments in declaration order.
• Kept def_to_js compatible by routing it through the same fragment registration path.
• Updated JS::Module output wrapping to use super(io) for compatibility with the new file-level composition flow.
• Added specs in spec/js/file/fragment_composition_spec.cr to cover:
• deterministic ordering of multiple fragments,
• reopen-style fragment contribution ordering,
• compatibility when combining js_function, js_fragment, and def_to_js.

Validation

• crystal tool format --check src spec
• CRYSTAL_CACHE_DIR=/tmp/.crystal-cache-js crystal spec (56 examples, 0 failures)