support for vue 3?

Nobody has claimed this yet.

Hi Linus.

I'm trying to use this plugin with a Vue 3 preview project created by Vue CLI, but when install and try run "serve" task this error message returns in terminal.

```
$ cross-env VUE_APP_BUILD_MODE=local vue-cli-service serve --port 8080 --mode development --dashboard
INFO Starting development server...

ERROR Error: Cannot find module 'vue-template-compiler/package.json'
Require stack:
- C:\Users\dian.cabral\Desktop\digital-rent-webapp\node_modules\vue-cli-plugin-test-attrs\index.js
- C:\Users\dian.cabral\Desktop\digital-rent-webapp\node_modules\@vue\cli-service\lib\Service.js
- C:\Users\dian.cabral\Desktop\digital-rent-webapp\node_modules\@vue\cli-service\bin\vue-cli-service.js

Error: Cannot find module 'vue-template-compiler/package.json'
Require stack:
- C:\Users\dian.cabral\Desktop\digital-rent-webapp\node_modules\vue-cli-plugin-test-attrs\index.js
- C:\Users\dian.cabral\Desktop\digital-rent-webapp\node_modules\@vue\cli-service\lib\Service.js
- C:\Users\dian.cabral\Desktop\digital-rent-webapp\node_modules\@vue\cli-service\bin\vue-cli-service.js
at Function.Module._resolveFilename (internal/modules/cjs/loader.js:880:15)
at Function.Module._load (internal/modules/cjs/loader.js:725:27)
at Module.require (internal/modules/cjs/loader.js:952:19)
at require (internal/modules/cjs/helpers.js:88:18)
at genVueLoaderCacheConfig (C:\Users\dian.cabral\Desktop\digital-rent-webapp\node_modules\vue-cli-plugin-test-attrs\index.js:63:32)
at C:\Users\dian.cabral\Desktop\digital-rent-webapp\node_modules\vue-cli-plugin-test-attrs\index.js:23:34
at C:\Users\dian.cabral\Desktop\digital-rent-webapp\node_modules\@vue\cli-service\lib\Service.js:236:40
at Array.forEach ()
at Service.resolveChainableWebpackConfig (C:\Users\dian.cabral\Desktop\digital-rent-webapp\node_modules\@vue\cli-service\lib\Service.js:236:26)
at Service.resolveWebpackConfig (C:\Users\dian.cabral\Desktop\digital-rent-webapp\node_modules\@vue\cli-service\lib\Service.js:240:48)
at PluginAPI.resolveWebpackConfig (C:\Users\dian.cabral\Desktop\digital-rent-webapp\node_modules\@vue\cli-service\lib\PluginAPI.js:132:25)
at serve (C:\Users\dian.cabral\Desktop\digital-rent-webapp\node_modules\@vue\cli-service\lib\commands\serve.js:75:31)
at Service.run (C:\Users\dian.cabral\Desktop\digital-rent-webapp\node_modules\@vue\cli-service\lib\Service.js:230:12)
at Object. (C:\Users\dian.cabral\Desktop\digital-rent-webapp\node_modules\@vue\cli-service\bin\vue-cli-service.js:36:9)
at Module._compile (internal/modules/cjs/loader.js:1063:30)
at Object.Module._extensions..js (internal/modules/cjs/loader.js:1092:10)

Total task duration: 25.11s
```

How to make it work? Thank you!!!

Start at the plugin entry point shown in the stack trace, index.js around line 63, and reproduce the reported vue-cli-service serve command in a Vue 3 CLI project. Trace why vue-template-compiler/package.json is required; done means the Vue 3 project starts without this missing-module error.

Written by the indexing model from the issue text.