Test `@extend` without a selector #1676

Job	Run time
Static analysis	17s
Unit tests	37s
Lint spec files	23s
Language \| Dart Sass \| Dart ${{ matrix.dart_channel }}	0s
JS API \| Pure JS \| Node ${{ matrix.node_version }} \| ${{ matrix.os }}	0s
JS API \| Pure JS \| Browser	0s
JS API \| Embedded \| Node lts/* \| ubuntu-latest	4m 51s
JS API \| Embedded \| Node lts/* \| windows-latest	7m 55s
JS API \| Embedded \| Node lts/* \| macos-latest	3m 41s
JS API \| Embedded \| Node lts/-1 \| ubuntu-latest	5m 0s
JS API \| Embedded \| Node lts/-2 \| ubuntu-latest	5m 3s
JS API \| Embedded \| Node lts/-3 \| ubuntu-latest	5m 17s
	33m 4s

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