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Fix Pynter worker crashing on missing node_module/document globals (#343)

@sourceacademy/pynter-wasm's generated glue calls
require("module").createRequire(import.meta.url) unconditionally at
module-load time — only the resulting require's *call* is guarded behind
an ENVIRONMENT_IS_NODE check, not its construction. In the browser IIFE
build this throws immediately when the worker script starts:
Rollup externalizes the Node builtin "module" as a bare global
(`node_module`) that's never defined, and its own import.meta.url
polyfill for IIFE output falls back to `document.baseURI`, which doesn't
exist in a Worker either. Either ReferenceError kills the whole worker
before the conductor's message channels are wired up, so nothing ever
reaches evaluateChunk()'s try/catch or surfaces to the UI — Pynter just
hangs forever with no output and no error, even with #339's separate
wasm-loading fix already deployed (verified live on sourceacademy.org).

Add a banner to the IIFE output defining both missing globals: a
createRequire stub that's safe to construct but never actually called,
and a document.currentScript stand-in pointing at the worker's own URL
(not document.baseURI — a blob: URL can't be used as a relative-resolution
base either, the same class of problem #339 hit).

Verified end-to-end with the real @sourceacademy/conductor host driving
the rebuilt bundle in an actual blob-URL Worker: print(1 + 2) now
completes with the correct output instead of hanging.

Claude-Session: https://claude.ai/code/session_01KrwJGug3rCXijRseRjys9V

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

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