Move the reserved-word, reserved-global, reserved-error-member, and
dunder exclusion sets from the worker backend up to the code-runtime
seam package, and narrow the portable identifier subset to drop the
JS-only `$`. Every backend now imports one contract so a binding
namespace list valid on one backend is valid on all.
Delivers only the seam extension and the worker's adoption; the Python
backend, py-types renderer, and Code Mode language dispatch are later
PRs in the stack that depend on these exports.
5d4cea9dc1