Allow passing mixed to assertVue and assertVueIsNot

[adi64]

This addresses the issues #845 and #850.
The assertVue (and assertVueIsNot) method have a DocBlock that declared that the expected value must be of type string. However, Vue component data can contain data of any type that JavaScript supports. The PHPUnit assertEquals check also supports these types.

I often want to use assertVue to check if arrays or objects in Vue are in the desired state. So far, there are many use cases (like empty arrays, arrays with nested arrays, or objects) that are not supported yet by the available assertions in Dusk.

This PR therefore allows passing mixed values to assertVue and assertVueIsNot. The error message is now set up by JSON-encoding the expected value.