Fix highlighting of interpolated strings in .html.erb files

In code like:

```html+erb
<a href="<%= link_to :whatever %>">
```

the scope that contains the ERB tags is a `.text .string` scope, not a `.source .string` scope like our styles were expecting.

We now don't care about any scopes outside of the string itself when detecting string interpolation. If there is source code inside a string, it is considered to be interpolation, no matter where that string lives.