feat(expressions): support assigment = operator

This adds support for the assignment operator (only `=`, no support for `+=` etc.). We need assignment operators in the `{let}` blocks. Additionally we might want to introduce them in expressions showing up in event handlers so one will be able to write sth like this:

```javascript
<a onclick="{ctrl.show = !ctrl.show}">Toggle me!</a>
```
I would only keep that one for `BINARY_OPERATORS`