Fix the behaviour of extending with as when mixing multiple values

[cristianbote]

Fixes #388

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[github-actions]

Size Change: +27 B (0%)

Total Size: 4.84 kB

Filename	Size	Change
dist/goober.esm.js	1.2 kB	+7 B (0%)
dist/goober.js	1.19 kB	+7 B (0%)
dist/goober.modern.js	1.2 kB	+7 B (0%)
dist/goober.umd.js	1.26 kB	+6 B (0%)

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