Bump mixin-deep from 1.3.1 to 1.3.2 in /webext (#286)

Bumps [mixin-deep](https://github.com/jonschlinkert/mixin-deep) from 1.3.1 to 1.3.2.
- [Release notes](https://github.com/jonschlinkert/mixin-deep/releases)
- [Commits](jonschlinkert/mixin-deep@1.3.1...1.3.2)

Signed-off-by: dependabot[bot] <[email protected]>