feat(webpack-5): opt-in to transitive only side effects

[sokra]

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update

Motivation / Use-Case

mini-css leaves empty "// extract by ..." js modules in the bundle. They are unnecessary.
webpack 5 will analyse the source code as side-effect-free, but as it contains a custom dependency (CssDependency), this dependency also need to opt-in into being side-effect-free. But actual it's not completely side-effect-free as it points to CssModules that need to be included in the bundle. So this PR opts in into a "transitive only" side-effects mode, which allows to drop the js module but keeps the dependencies in the bundle.

Breaking Changes

None

Additional Info

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@ef4bd79). Click here to learn what that means.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #599   +/-   ##
=========================================
  Coverage          ?   88.00%           
=========================================
  Files             ?       10           
  Lines             ?      600           
  Branches          ?      155           
=========================================
  Hits              ?      528           
  Misses            ?       69           
  Partials          ?        3           

Impacted Files	Coverage Δ
src/CssDependency.js	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ef4bd79...0d8f925. Read the comment docs.

[l5oo00]

This feature may make the effect of optimization.chunkIds: "deterministic" not as expected.

The example repository: https://github.com/l5oo00/demo-for-mini-css-extract-plugin