build

Implement .to_s for Sass::Value::String #1988

Job	Run time
spec (alpine-latest, 3.1)	31s
spec (alpine-latest, 3.2)	35s
spec (alpine-latest, 3.3)	52s
lint	10s
spec (macos-latest, 3.1)	34s
spec (macos-latest, 3.2)	30s
spec (macos-latest, 3.3)	1m 14s
spec (macos-latest, jruby)	1m 31s
spec (macos-latest, truffleruby)	1m 43s
spec (macos-latest, truffleruby+graalvm)	5m 51s
spec (ubuntu-latest, 3.1)	21s
spec (ubuntu-latest, 3.2)	17s
spec (ubuntu-latest, 3.3)	34s
spec (ubuntu-latest, jruby)	1m 29s
spec (ubuntu-latest, truffleruby)	1m 13s
spec (ubuntu-latest, truffleruby+graalvm)	4m 55s
spec (windows-latest, 3.1)	2m 12s
spec (windows-latest, 3.2)	3m 24s
spec (windows-latest, 3.3)	3m 51s
spec (windows-latest, jruby)	3m 44s
release	5s
	35m 36s

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