feat: support for messaging from main world content scripts

[edtervit]

Details

This PR ...

Allows usage of the message API when the content script is in the main world.

To send messages from content scripts in the main world chrome requires the following:

• The extension's id to be passed as a string to sendMessage()
• The service worker's listener to use onMessageExternal() instead of onMessage()

Read details from chrome here

I also submitted a PR to the docs that explains this change

This is my first proper PR to open source so please be kind ❤️

Code of Conduct

• I agree to follow this project's Code of Conduct
• I agree to license this contribution under the MIT LICENSE
• I checked the current PR for duplication.

[louisgv]

@edtervit thank you for the PR :) - Are we sure that by passing null to the first argument, it would behave the same as without it?? I'm surprised this is the behavior lol!

[louisgv]

Holy cow you're right - I've never seen a pattern where the 1st argument is optional lol....

[louisgv]

LGTM 👍 - note that this is a substantial API behavior change.

If the user doesn't need to use external message, it would duplicate the generated code. However, it's prob not a huge deal since the switch statement is pretty compact anyway.