`provide` function unavailable in Firefox content scripts context

Our team is trying to write a browser extension to create an autocomplete search feature inside Github for a 3rd party issue tracking system.

We almost have it working on Chrome, but on Firefox the [content security limitations of content scripts](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Sharing_objects_with_page_scripts#xray_vision_in_firefox) is preventing us from making this work.

The issue is that the [`provide`](https://github.com/github/text-expander-element/blob/main/src/text-expander-element.ts#L250-L252) function sent in the detail object of the `text-expander-change` `CustomEvent` is not accessible inside the Firefox content script.

We would appreciate any refactor that would allow us to use this component without any passthrough functions or DOM fragments.