#1621 Modernize Handlebars and improve security documentation in sandbox sample

[Anonymous]

Originally created by: RinZ27

Addressing the discussion in [#1615] regarding modernized dependencies and security clarity.

Changes

• Handlebars Upgrade: Updated the sandboxed Handlebars library from 1.0.0.beta.6 to 4.7.8 (minified). This addresses potential legacy vulnerabilities and brings the sample up to modern standards.
• Security Documentation: Added a comment in mainpage.js explaining the use of * as targetOrigin in postMessage. As discussed, the chrome-extension:// scheme is not supported by the web-standard postMessage API for origin filtering, making the wildcard necessary for extension-to-sandbox communication.

These updates ensure the sample remains functional while providing better technical context for developers.

Fixes [#1615] (partially, regarding Handlebars and documentation).