[javascript] template strings are not correctly parsed

**Rule Set:** N/A

**Description:** ECMAScript 2015 (ES6) added support for template strings, delimited by backsticks (`` ` ``). The current grammar does not support this properly, meaning template strings throw lexical errors.

**Code Sample demonstrating the issue:**

```
var markup = `<p>${content}</p>`;
```

**Running PMD through:** *Any*

