fix: turn on ts-eslint version of no-unused-vars for ts & vue files

Fixes #14
Fixes vuejs/create-vue#158

Technically turning new rules on would be a breaking change because
it introduces more errors in an existing codebase.
However, in our README we've already made it clear that this ruleset
should only be used with `@vue/cli` or `create-vue`, so an existing
`eslint:recommended` config is expected, thus no breaking changes.

Nevertheless, I choose to make it `warn` instead of `error` to minimize
the impact.

index.js

@@ -27,11 +27,12 @@ module.exports = {
 
   overrides: [
     {
-      files: ['*.ts', '*.tsx'],
+      files: ['*.ts', '*.tsx', '.vue'],
       rules: {
         // The core 'no-unused-vars' rules (in the eslint:recommeded ruleset)
         // does not work with type definitions
         'no-unused-vars': 'off',
+        '@typescript-eslint/no-unused-vars': 'warn'
       }
     }
   ]