Add webpack as a peer dependency to silent unmet peer dependency warnings

[murny]

Cleans up these kind of warnings:

[1/5] :mag:  Resolving packages...
[2/5] :truck:  Fetching packages...
[3/5] :link:  Linking dependencies...
warning " > [email protected]" has unmet peer dependency "webpack@^2.0.0 || ^3.0.0 || ^4.0.0".
warning " > [email protected]" has unmet peer dependency "webpack@^4.0.0 || ^5.0.0".
warning "webpack-dev-server > [email protected]" has unmet peer dependency "webpack@^4.0.0".

Webpacker has a dependency already on webpack so these warnings are harmless. But we get them as the above packages want webpack to be a peer dependency (webpack be added to our projects dependencies). So let's just silent them by adding webpack to our project using resolutions.

More info from webpack: rails/webpacker#1078