fix: allow run in service worker

[alanshaw]

Before creating a pull-request, please check https://github.com/handlebars-lang/handlebars.js/blob/master/CONTRIBUTING.md first.

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• Please don't start pull requests for security issues. Instead, file a report at https://www.npmjs.com/advisories/report?package=handlebars
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[jaylinski]

Discussion in #1894.

[jaylinski]

Fixed in #1894.