Relax rule: Allow tagged template string expressions (no-unused-expressions)

Fixes: standard/standard#822

Fixes "Expected an assignment or function call and instead saw an
expression." error when using a standalone tagged template string like:

```js
var css = require('sheetify')

css`
  .js-yellow { color: #f7df1e }
  .bg-js-yellow { background-color: #f7df1e }
`
```

Author: feross

eslintrc.json

@@ -133,7 +133,7 @@
     "no-unreachable": "error",
     "no-unsafe-finally": "error",
     "no-unsafe-negation": "error",
-    "no-unused-expressions": ["error", { "allowShortCircuit": true, "allowTernary": true }],
+    "no-unused-expressions": ["error", { "allowShortCircuit": true, "allowTernary": true, "allowTaggedTemplates": true }],
     "no-unused-vars": ["error", { "vars": "all", "args": "none", "ignoreRestSiblings": true }],
     "no-use-before-define": ["error", { "functions": false, "classes": false, "variables": false }],
     "no-useless-call": "error",