Specify webpack-dev-server to be v3

[adrianbautista]

Otherwise when using Webpacker v5, it will install webpack-dev-server v4 which then requires @webpack-cli/serve to also be installed to properly run bin/webpack-dev-server.

When using Webpacker v5 with Webpack v4 and trying to run bin/webpack-dev-server:

$ bin/webpack-dev-server
The command moved into a separate package: @webpack-cli/serve
Would you like to install serve? (That will run npm install -D @webpack-cli/serve) (yes/NO) :

[DavidColby]

Would it be possible to get a new release out with this change?

Creating a new Rails 6.1 application results in a broken webpack-dev-server since the latest release of webpacker (5.4.2) does not include the change in this PR. Figuring out what's going on with the error loop can be tricky for folks who are new to Rails or who are coming to Rails 6 for the first time from an older, webpacker-less version of Rails.

Recreate the issue with:

rails new some_new_app
bin/webpack-dev-server # Enter the error loop this PR fixes

[dhh]

Yeah, should do that.