Bump shiki from 1.17.6 to 1.21.0

Bumps [shiki](https://github.com/shikijs/shiki/tree/HEAD/packages/shiki) from 1.17.6 to 1.21.0.
- [Release notes](https://github.com/shikijs/shiki/releases)
- [Changelog](https://github.com/shikijs/shiki/blob/main/CHANGELOG.md)
- [Commits](https://github.com/shikijs/shiki/commits/v1.21.0/packages/shiki)

---
updated-dependencies:
- dependency-name: shiki
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

package.json

@@ -83,7 +83,7 @@
     "rfdc": "^1.4.1",
     "rimraf": "^6.0.1",
     "serve-handler": "^6.1.5",
-    "shiki": "^1.17.6",
+    "shiki": "^1.21.0",
     "source-map-support": "^0.5.21",
     "to-vfile": "^8.0.0",
     "toml": "^3.0.0",