build(deps): bump postcss and web-ext (#427)

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.32 and
updates ancestor dependency
[web-ext](https://github.com/mozilla/web-ext). These dependencies need
to be updated together.

- Updates `postcss` from 8.4.29 to 8.4.32
- Updates `web-ext` from 7.6.2 to 7.9.0

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