Update dependency webextension-polyfill to ^0.12.0

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
webextension-polyfill	^0.11.0 -> ^0.12.0

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Release Notes

mozilla/webextension-polyfill (webextension-polyfill)

### [`v0.12.0`](https://togithub.com/mozilla/webextension-polyfill/releases/tag/0.12.0) [Compare Source](https://togithub.com/mozilla/webextension-polyfill/compare/0.11.0...0.12.0) ##### Bug Fixes - Removed unnecessary webpack and webpack-cli npm packages from webextension-polyfill npm package dependencies ([#​614](https://togithub.com/mozilla/webextension-polyfill/issues/614)) ([https://github.com/mozilla/webextension-polyfill/issues/604](https://togithub.com/mozilla/webextension-polyfill/issues/604)) See [all changes for 0.12.0](https://togithub.com/mozilla/webextension-polyfill/compare/0.11.0...0.12.0) - https://unpkg.com/webextension-polyfill@0.12.0/dist/

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