fix(vitest): add @vitest/coverage-v8 and @vitest/coverage-istanbul as optional dependency

The `@vitest/coverage-v8` and `@vitest/coverage-istanbul` package is now listed as an optional peer dependency. This allows users to opt-in to v8 coverage without requiring it as a hard dependency, while satisfying package manager hoisting and linking requirements.

Author: alan-agius4

packages/vitest/package.json

@@ -139,6 +139,8 @@
     "@vitest/browser-playwright": "workspace:*",
     "@vitest/browser-preview": "workspace:*",
     "@vitest/browser-webdriverio": "workspace:*",
+    "@vitest/coverage-istanbul": "workspace:*",
+    "@vitest/coverage-v8": "workspace:*",
     "@vitest/ui": "workspace:*",
     "happy-dom": "*",
     "jsdom": "*",
@@ -163,6 +165,12 @@
     "@vitest/browser-webdriverio": {
       "optional": true
     },
+    "@vitest/coverage-istanbul": {
+      "optional": true
+    },
+    "@vitest/coverage-v8": {
+      "optional": true
+    },
     "@vitest/ui": {
       "optional": true
     },