Improve diff-blob-expander-icon

[simurai]

This addresses https://github.com/github/design-infrastructure/discussions/1439#discussioncomment-1714926 by improving contrast of that expander icon:

Before

After

Light High Contrast	Dark Dimmed
fg.onEmphasis	fg.default

[changeset-bot]

🦋 Changeset detected

Latest commit: 028d9fa

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@primer/primitives	Patch

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[github-actions]

Variables changed

--- base/dist/scss/colors/_light_high_contrast.scss	2021-11-30 08:25:06.964686641 +0000
+++ dist/scss/colors/_light_high_contrast.scss	2021-11-30 08:24:56.672464660 +0000
@@ -16,3 +16,3 @@
     --color-diff-blob-hunk-num-bg: #9cd7ff;
-    --color-diff-blob-expander-icon: #0E1116;
+    --color-diff-blob-expander-icon: #ffffff;
     --color-diff-blob-selected-line-highlight-mix-blend-mode: multiply;