fix: set args to after-used for no-unused-vars rule (#103)

This PR makes the following code failing linting. IMO this helps with
refactoring and making sure arguments are not left in function calls.

```js
// Lint error: "'c' is defined but never used."
function unused (a, b, c) {
  return b
}
unused()
```

lib/index.js

@@ -161,7 +161,7 @@ module.exports = {
     'no-unused-expressions': ['error', {
       allowShortCircuit: true, allowTernary: true, allowTaggedTemplates: true,
     }],
-    'no-unused-vars': ['error', { vars: 'all', args: 'none', ignoreRestSiblings: true }],
+    'no-unused-vars': ['error', { vars: 'all', args: 'after-used', ignoreRestSiblings: true }],
     'no-use-before-define': ['error', { functions: false, classes: false, variables: false }],
     'no-useless-call': 'error',
     'no-useless-computed-key': 'error',

test/index.js

@@ -1,23 +1,40 @@
 const { ESLint: ESLint8 } = require('eslint')
 const t = require('tap')
 
-const code = `const foo = 1
+const loadConfig = (desc, Ctor) => new Ctor({
+  useEslintrc: false,
+  overrideConfigFile: 'lib/index.js', // this is relative to the root of the repo
+})
+
+const lintText = async (engine, code) => {
+  const [result] = await engine.lintText(code)
+  return result
+}
+
+t.test('eslint8', async t => {
+  const engine = loadConfig('eslint 8', ESLint8)
+
+  t.test('config loads correctly', async t => {
+    const code = `const foo = 1
 const bar = function () {}
 bar(foo)
 `
-
-t.test('config loads correctly', t => {
-  const loadConfig = (desc, Ctor) => t.test(async t => {
-    const engine = new Ctor({
-      useEslintrc: false,
-      overrideConfigFile: 'lib/index.js', // this is relative to the root of the repo
-    })
-
-    const [result] = await engine.lintText(code)
+    const result = await lintText(engine, code)
     t.equal(result.errorCount, 0, 'no errors')
   })
 
-  t.plan(1)
-  loadConfig('eslint 8', ESLint8)
-  t.end()
+  t.test('unused args', async t => {
+    const code = `function unused (a, b, c) {
+  return b
+}
+function allUsed (a, b) {
+  return b
+}
+unused()
+allUsed()
+`
+    const result = await lintText(engine, code)
+    t.equal(result.errorCount, 1, 'no errors')
+    t.equal(result.messages[0].message, "'c' is defined but never used.")
+  })
 })