spec: update codegen-js.md to Phase 5, ~90% coverage
Status updated from Phase 4 ~80% to Phase 5 ~90%.
New sections:
- 7. Language features coverage table (supported vs stubbed)
- 7a. Phase 5 constructs: extern fn, anonymous functions, try/catch,
method call on Any, URL imports -- each with emit shape examples
- 9. Roadmap updated: Phases 1-5 marked DONE, Phase 6 unblocked
Runtime builtin table updated to ~90 builtins including all Phase 5
additions (promise_then/catch/resolve/reject, object_assign/keys/values,
json_deep_clone, array_from, type_of, instanceof_check).
Will Anderson
e858eab300