Add a note about chilled string support to 3.1.8 release note #527

Job	Run time
ruby-versions / Generate Ruby versions	5s
windows-latest 3.0	1m 3s
windows-latest 3.1	1m 11s
windows-latest 3.3	1m 0s
windows-latest 3.4	55s
windows-latest mingw	54s
windows-latest 2.7	1m 35s
windows-latest mswin	1m 16s
windows-latest 3.2	1m 1s
windows-latest jruby-head	1m 45s
	10m 45s

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