test: update tests for Vue 2.7

- TypeScript version is updated because `vue/types/jsx.d.ts` doesn't
work with TypeScript 3.9
- Scoped CSS specs are updated because `vue/compiler-sfc` in 2.7
uses `shortId` instead of full id in keyframes https://github.com/vuejs/vue/blob/9dd006b481b4299462e044741bac0861c0b1775c/packages/compiler-sfc/src/stylePlugins/scoped.ts#L9
- source map tests is moved to a separate file because it seems to be
disturbed by other test cases in `advanced.spec.js`

package.json

@@ -85,12 +85,13 @@
     "stylus": "^0.54.5",
     "stylus-loader": "^3.0.2",
     "sugarss": "^1.0.1",
+    "thread-loader": "^3.0.4",
     "ts-loader": "^4.2.0",
-    "typescript": "^3.9.5",
+    "typescript": "^4.8.4",
     "url-loader": "^1.0.1",
-    "vue": "^2.5.16",
-    "vue-server-renderer": "^2.5.16",
-    "vue-template-compiler": "^2.5.16",
+    "vue": "^2.7.14",
+    "vue-server-renderer": "^2.7.14",
+    "vue-template-compiler": "^2.7.14",
     "vuepress": "^0.14.2",
     "vuepress-theme-vue": "^1.1.0",
     "webpack": "^4.46.0",