Update Alpha Color Tokens (#2635)

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## Self Checklist

- [x] I wrote a PR title in **English** and added an appropriate
**label** to the PR.
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about the changes that needed to be released. (or didn't have to)
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## Related Issue

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<!-- Fixes #0000 -->

## Summary

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변경된 요구사항에 맞게 알파 컬러 토큰을 업데이트합니다

## Details

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생략

### Breaking change? (Yes/No)

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NO

## References

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Author: sungik-choi

.changeset/wet-swans-report.md

@@ -0,0 +1,5 @@
+---
+'@channel.io/bezier-tokens': patch
+---
+
+Updated surface colors in dark theme and Added grey-950(#1a1a1c) color to the global color palette.

packages/bezier-tokens/src/alpha/functional/dark-theme/color.json

@@ -820,15 +820,15 @@
     },
     "surface": {
       "lowest": {
-        "value": "{color.grey.800}",
+        "value": "{color.grey.850}",
         "type": "color"
       },
       "lower": {
-        "value": "{color.grey.850}",
+        "value": "{color.grey.900}",
         "type": "color"
       },
       "normal": {
-        "value": "{color.grey.900}",
+        "value": "{color.grey.950}",
         "type": "color"
       }
     },

packages/bezier-tokens/src/alpha/global/color.json

@@ -786,6 +786,10 @@
       "900-0": {
         "value": "#24242800",
         "type": "color"
+      },
+      "950": {
+        "value": "#1a1a1c",
+        "type": "color"
       }
     },
     "grey-alpha": {