Bump webext-messenger from 0.20.1 to 0.21.0-0

[fregante]

What does this PR do?

• Includes https://github.com/pixiebrix/webext-messenger/pull/84

Discussion

• It shouldn't have any real user-facing changes except better errors. I just need to try to cause these errors and see if we were catching the old errors somewhere.

Checklist

• [x] Test
• [x] Designate a primary reviewer: @twschiller

[fregante]

How to see these new error messages:

The target ${JSON.stringify(target)} for ${type} was not found

1. Double-click the browser action

We already always wrap this message so the user doesn't see it, but it can appears on Rollbar.

https://github.com/pixiebrix/pixiebrix-extension/blob/a6aa32e04867a5ebb273c93e8c8755ae1040944c/src/contentScript/sidebarController.tsx#L70-L74

Messenger was not available in the target ${JSON.stringify(target)} for ${type}

This should never appear on production, it means we didn't load the Messenger.

Conflict: The message ${type} was handled by a third-party listener

This should never appear on production, it means that either:

• we're using chrome.runtime.onMessage.addListener directly and incorrectly
• one of our dependencies is doing that (for example https://github.com/awesome-webextension/webpack-target-webextension/blob/860688df0f22bc367305254e2803fa80f8efdf11/lib/webpack5/RuntimeModules/ChunkLoaderFallback.js#L27)

[fregante]

As tested, this change doesn't affect the user and can be merged as is.

[codecov-commenter]

Codecov Report

Merging #4169 (2aaed9c) into main (a6aa32e) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #4169   +/-   ##
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  Coverage   48.57%   48.57%           
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  Files         877      877           
  Lines       26150    26150           
  Branches     5397     5397           
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  Hits        12703    12703           
  Misses      12519    12519           
  Partials      928      928           

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