Test `@extend` without a selector #1677

Job	Run time
Language \| Dart Sass \| Dart ${{ matrix.dart_channel }}	0s
JS API \| Pure JS \| Browser	0s
JS API \| Pure JS \| Node ${{ matrix.node_version }} \| ${{ matrix.os }}	0s
JS API \| Embedded \| Node lts/* \| ubuntu-latest	4m 52s
JS API \| Embedded \| Node lts/* \| windows-latest	7m 49s
Lint spec files	15s
Static analysis	16s
JS API \| Embedded \| Node lts/* \| macos-latest	3m 45s
Unit tests	38s
JS API \| Embedded \| Node lts/-1 \| ubuntu-latest	4m 51s
JS API \| Embedded \| Node lts/-2 \| ubuntu-latest	5m 8s
JS API \| Embedded \| Node lts/-3 \| ubuntu-latest	5m 18s
	32m 52s

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