Modernizing the sandbox sample: Handlebars update and messaging hardening

Refining the sandbox sample to reflect modern standards would be beneficial for developers learning from this repo. Noticed that the api-samples/sandbox directory still relies on Handlebars 1.0.0-beta.6, which dates back to 2012. Using such an ancient version in a current sample might lead developers to inadvertently include insecure or unmaintained dependencies in their own extensions.

Tightening the communication logic by replacing postMessage("*") with an explicit origin check would also set a better security precedent. Even though the sandbox environment provides isolation, showcasing a restricted origin check helps prevent developers from copy-pasting wildcard patterns into non-sandboxed contexts.

Would you be open to a PR that updates the library to a stable version and implements more restrictive messaging? Just wanted to check if this is something you'd like to see updated to keep the samples aligned with current best practices.