feat!: update to Vue 2.7

Known caveats:
1. JSX not yet supported
2. Though `vue-template-compiler` is not required in most cases,
`@vue/test-utils` still requires it as a peer dependency.
3. `IconTooling.vue` was modified to circumvent a type-checking issue:
vuejs/language-tools#1526

Author: haoqunjiang

.github/workflows/ci.yml

@@ -33,6 +33,7 @@ jobs:
         with:
           node-version: ${{ matrix.node-version }}
           cache: 'pnpm'
-      - run: pnpm install
+      - run: pnpm install --no-frozen-lockfile
       - run: pnpm pretest
+      - run: pnpm install --no-frozen-lockfile
       - run: pnpm test