feat(external_messaging): Adds external messaging resolver

[sleekslush]

Details

Introduces concept of external messaging handlers. By using the path of messages/external/, we can leverage the same messaging features, but they will be registered via chrome.runtime.onMessageExternal.addListener instead of chrome.runtime.onMessage.addListener.

This fixes an issue currently where all message handlers are being registered as internal AND external handlers, which can be unsafe.

You can either set up a message handler in background/messages/my-handler.ts , or background/messages/external/my-external-handler.ts. If you want a message handler to work in both contexts, symlink it ln -s background/messages/my-safe-handler.ts background/messages/external/my-safe-handler.ts.

Code of Conduct

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• I agree to license this contribution under the MIT LICENSE
• I checked the current PR for duplication.

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[louisgv]

LGTM 👍

[ivliag]

The thing to notice - this actually breaks backwards compatibility, as currently registered external message handlers will stop working until moved to messages/external.