migrate to webpack 5

[dmanto]

These changes will allow to work with webpack 5+, which I think should be the new default.

webpack 5 introduced a lot of breaking changes, I just fixed the minimum to pass current test, specially t/example-build-assets.t test.

They are big changes though, and I am not very sure of the full implicances (I am not an expert on webpack, but I have been using it since several months). I have a version running in my computer with these changes and running webpack 5, and seem to work fine, but other than that I can't tell.

There is also a specific guide on webpack site, that can address further needed changes, but I didn't need to follow it because the changes introduced by this PR were enough in my case.

Some breaking changes considered:

• I needed to specify @next on html-webpack-plugin, otherwise it install the former version (for webpack 4)
• terser-webpack-plugin doesn't support cache nor sourceMap options, so I removed those
• optimize-css-assets-webpack-plugin is not supported anymore. I used css-minimizer-webpack-plugin to replace it
• contenthash seem to be generated ok without webpack-plugin-hash-output, so I removed it because caused lots of warnings

Quick testing

It can be dificult to migrate from a installed webpack 4 to 5 in the middle of a project, so in my repo there is a branch 'test-container' that can be used as a smoke test if you change anything. Has a minimal Dockerfile to have a perl 5.32 container with npm installed.

Just checkout to that branch and run:

docker build -t test-cont . && docker run -e TEST_ALL=1 -t test-cont prove -l t/example-build-assets.t

[jhthorsen]

Looks great!

The description is excellent and the changes are to the point. Thank you 👍

The plugin will not change an existing package.json, so projects that use webpack 4 will just continue to work.