Add dedicated element pickers for "strong"-tag to the DSL

[skrapeit]

currently it is possible to to pick strong-tag using

...
expect {
    element("strong") {
        text() toBe "I'm the inner text"
     }
}

make it possible to have a more convenient and type safe way of picking elements.

e.g.:

...
expect {
    strong {
        text() toBe "I'm the inner text"
     }
}