lint: switch to using eslint directly

* this allows the use of the --cache flag for faster repeated linting (not
  supported in `vue-cli-service lint`)
  see: https://cli.vuejs.org/core-plugins/eslint.html#injected-commands
* `vue-cli-service` is deprecated, and using `eslint` directly is recommended
  for vue 3
  see: https://cli.vuejs.org/core-plugins/eslint.html
  see: https://vuejs.org/guide/scaling-up/tooling.html#linting

.gitignore

@@ -30,3 +30,5 @@ npm-debug.log*
 
 /.nginx/*
 !/.nginx/.gitkeep
+
+/.eslintcache