infra: move c8 (coverage) to devDeps (#743)

- used only in tests, but caused downstream to install it as well, raising the number of deps.
- also bumped version to 7.12.0 (latest)

package.json

@@ -39,7 +39,6 @@
     "ajv": "8.11.0",
     "arg": "5.0.2",
     "boxen": "7.0.0",
-    "c8": "7.11.3",
     "chalk": "5.0.1",
     "chalk-template": "0.4.0",
     "clipboardy": "3.0.0",
@@ -52,6 +51,7 @@
     "@types/compression": "1.7.2",
     "@types/serve-handler": "6.1.1",
     "@vercel/style-guide": "3.0.0",
+    "c8": "7.12.0",
     "eslint": "8.19.0",
     "got": "12.1.0",
     "husky": "8.0.1",