test: cover more module types with the built-in CSS support

[alexander-akait]

Summary

Follow-up to #609, rebased on top of it. The README rewrite and the tests that #609 already landed are dropped from this branch, leaving only the additional coverage.

getCssCompiler hardcoded type: "css/auto" on the rule, so only that module type could be exercised. The helper now takes the module type from its config argument (still defaulting to css/auto), which makes the other types testable.

New tests

• css type — a *.module.less file compiled with type: "css" keeps its local class names, i.e. it is not treated as a CSS module because of its name.
• css/module type — a file without .module in its name compiled with type: "css/module" has its local class names renamed.
• Less warnings — a Less warning is reported as a webpack warning under the built-in CSS support (docs: use the built-in CSS support of webpack in examples #609 covers errors, not warnings).

The suite is green at 143 tests, as are npm run lint and npm run build.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 2141570

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.61%. Comparing base (85812ac) to head (2141570).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #610   +/-   ##
=======================================
  Coverage   96.61%   96.61%           
=======================================
  Files           2        2           
  Lines         649      649           
=======================================
  Hits          627      627           
  Misses         22       22           

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