fix(cli-plugin-eslint): make eslint deps optional (#3068)

1. In @vue/cli-plugin-eslint 3.1.0+, eslint will be explicitly added to
the project's package.json, so it is **optional**.
2. `optionalDependencies` will still be installed by npm by default,
just no longer cause installation to fail when error happens.
3. ESLint v4 did cause weird errors sometimes, like `npm WARN
ajv-keywords@2.1.1 requires a peer of ajv@^5.0.0 but none is installed.`
Though this warning never caused any errors on my machine, I think it's
good to make the plugin fail-safe.

packages/@vue/cli-plugin-eslint/package.json

@@ -25,10 +25,12 @@
   "dependencies": {
     "@vue/cli-shared-utils": "^3.4.0",
     "babel-eslint": "^10.0.1",
-    "eslint": "^4.19.1",
     "eslint-loader": "^2.1.1",
-    "eslint-plugin-vue": "^4.7.1",
     "globby": "^9.0.0",
     "webpack": ">=4 < 4.29"
+  },
+  "optionalDependencies": {
+    "eslint": "^4.19.1",
+    "eslint-plugin-vue": "^4.7.1"
   }
 }

packages/@vue/cli-service-global/package.json

@@ -28,6 +28,8 @@
     "@vue/cli-service": "^3.4.0",
     "babel-eslint": "^10.0.1",
     "chalk": "^2.4.2",
+    "eslint": "^4.19.1",
+    "eslint-plugin-vue": "^4.7.1",
     "resolve": "^1.10.0",
     "vue": "^2.5.22",
     "vue-template-compiler": "^2.5.22"