qmlls: scriptexpression support for go-to definition

Implement scriptexpression support for go-to definition.

Example from Fabian:

Item {
    property int i // (1)
    function f(a /*(2)*/ , b) {return a /*  go to definition on a leads to (2) */  > b};  // (4)
    Component.onCompleted: {
        let x = 42 (3)
        f(x, i) // goto definition on f goes to 4, on x goes to (3) and on i goes to (1)
    }
}

Note from Fabian:

The QMLDOM is completely unaware of what's in script expressions so far – you might need to get access to the QQmlJSScope of the file for doing resolution in there. Given that we run the linter already, you should however be able to get access to a valid scope.